Co-Written by AustralianBeast17

Four weeks had passed ever since Bart had convinced Lisa to be happy about having a phat ass and how it would be great to use it for twerking, now she wasn't ashamed of it anymore, they would also have some private brother and sister bonding time that no one else was allowed to have if Bart had somehow gone further with her besides a buttjob it was yet to be seen. Lisa's whole wardrobe had changed though, now she enjoyed wearing skirts, short shorts, and all kinds of clothing that would make her butt look great, just to tease her brother. Even when there were other people in the room, all she had to do was act innocent and naive. Bart hated\loved her doing that and begged for a chance for them to be alone.

Did anyone else notice the change in their behavior? Well yes, and it was their mom, but Marge also noticed something weird, even though the two siblings seemed closer and it made her happy, there was something else. She noticed that her daughter constantly seemed to drop things by accident on the floor only to bend over in front of Bart, and how close they would sit on the couch or the dinner table. It wasn't normal for Brother and Sister to be so casual like this, right?

"OOOPPPHH!" Marge gasped out from her thoughts when a certain "asset" of hers got stuck between the door. "Oh, dear...Not again!"

During the past years, the blue-haired MILF has noticed that her figure has dramatically changed during the past years. It was ridiculously curvy at this point, her tits so large that six of her own heads couldn't fully account for the volume of her massive bust, her ass so gigantic she experienced some trouble leaving her room to make her way into the hallway, and to make it worse, her hips were so wide that she almost wobbled with every step. Her hips were perfect for breeding, however, due to her recent changes, they were an enigma to Marge making walking somewhat of a challenge.

"Honestly, sometimes I wonder why I hit the sudden growth spurt, wasn't that meant to happen in teen years?"

Marge forces herself to get out of that situation, using all her strength to finally free her big assets away from the door. Now Marge knew who Lisa got her derriere from, she thought it wasn't her when it happened but now? Maybe the girls in her family were just meant to be bottom-heavy.

"I have to walk around with butter or oil if I have to be stuck like this every damn time"

Grumbling about the fact the massive curves are messing with her, Marge made her way to the hallway where Homer was preparing to leave with a large suitcase while he held the 6-year old Maggie in his arms and blew raspberries at her belly, which caused her to giggle.

"Homey? Where are you going right now?" Marge asked.

"Hmm? Oh sorry, honey, didn't I tell you? A business trip has arrived and old burns want me to come along for some stupid, I don't know"

"I see...Well, I hope you have a fun journey, Homey!" Marge cheerfully wished her husband luck with his trip while giving him a deep kiss on the lips to which he passionately returned. However, their kissing was interrupted when Maggie, who was still in her daddy's arms, gagged at the sight.

"Eeeeewwww, Mama and Papa kissing! Gross!" Maggie squealed as she looked away from the icky scene.

"Oh, Maggie...My little princess! I'm gonna miss you so much!" Homer told Maggie with a sweet voice as he put her down and kissed her forehead. As Maggie ran off to play with her toys, Homer saw that Bart and Lisa made their way towards him as well.

"Bye, dad! Make sure not to do anything embarrassing again." Lisa said while she hugged her father goodbye.

"Hehehe...Can't make any promises, Lisa!" Homer told his daughter in a jolly voice while he ruffled her hair, much to her own embarrassment.

"He sure can't...Especially when he will eat like a big fat pig at the all-you-can-eat buffet." Bart remarked while having his hands in his pockets.

"WHY YOU LITTLE...!" Homer growled in fury before he rushed towards Bart and started to choke him with his bare hands! Bart's eyes literally popped out of his head when his father kept on gripping his throat in two!

Ah yes, just a normal loving looking family, sure it was rough around the edges with their little quirks, but Marge loved having them. It would be a bit lonely without her Homey but way more silenced now and with less screaming, would the next few days be calmer, or was this just the start of something far more concerning?

*Later that day...*

"Maggie is busy playing with her toys upstairs, but where are Lisa and Bart? I have to ask them to do something for me"

Going upstairs, Marge made her way to find her son and daughter. However, due to her rather exaggerated proportions, more than once there was an audible clap coming from the blue-haired MILF's more than sizable mountainous ass and colossal tits when they collided with almost every step she took. Embarrassed, Marge took her hands over her gigantic phat rump to try and quiet the sound of her ass clapping as best as she could.

'Stupid phat ass, making me embarrassed every time I walk, maybe I should start wearing jean pants'

Something took Marge out of her musing because there was a certain noise coming from Bart's room, and feeling curious about it because her special little guy is always making stuff that ends in pranks, she decided to go check it out in case she needs to butt in as her job as a mother. The blue-haired mother began to tiptoe towards Bart's door while making sure neither her massive udders nor her enormous ass stayed quiet. Slowly opening the door, Marge peeked inside of the room through the keyhole, and the sight she witnessed caused her to gasp out in shock and horror.

'OH...MY...GOD...!' She thought, trying to hold down her voice.

On the other side of that door, something impossible was happening, or at least something that wasn't supposed to happen. Lisa Simpson, her daughter, was wearing a short skirt and was shaking her big rump up and down close to the crotch of no one else but her brother! Bart Simpson, who seemed to be enjoying it, showing a wide grin.

"Damn sis, you have gotten so much better at this, impressive"

"T-T-Thanks, B-Bart...I've been training these past 4 weeks just for you!" Lisa panted while trying to hold down her moans as she grinded and shook her huge rump on her brother's crotch.

"You have been teasing me constantly as well Lisa, it seems you're seeking my attention huh?" He said, using one hand to rub her ass cheek, feeling how big and soft it was.

"God, Lisa...You really have one big sexy ass...Not as gigantic or phat as mom's ass...But you'll get there one day!" Bart said with a lecherous smile as he massaged Lisa's soft round ass before he raised his hand up and began to repeatedly slap it!

"AHN!" She let out a moan.

"Shhh, not too loud my bootylicious sister, you don't want anyone seeing you in this predicament right? Hehe," Bart smirked.

"M-Meanie..." Lisa pouted in pain and she moaned once again as Bart continued to mercilessly slap her sore buttcheeks as they bounced from each strike that struck them. The oldest of the Simpson children laughed as he continued to spank his sister's huge butt to his aroused amusement.

Directly outside of Bart's room, however, neither of the siblings was aware that their mother was peeping on them in silent shock. But Marge's shock would turn into a rage as she slammed open the door in fury which started both of the siblings!

"WHAT THE HELL IS GOING ON HERE?!" Marge roared with rage and disgust, while unaware that her massive curves bounced from her outburst with her mega-sized naked tits popped out and the lower part of her dress rode up between her buttcheeks, showing off more of her absurdly defined, extremely fat bubble butt to the entire world.

"GAH!" Bart finally saw he was busted.

"M-Mom? It's not what it looks like" Lisa tried to defend herself.

"Not what it looks like? I leave for one moment and when I come back I see my daughter twerking for her brother like it's so normal?" Marge glared at them.

"NO, NO, NO, NO, MOM! You've got it all wrong!" Bart nervously said as he tried to explain to his furious mother while he nervously pulled on his collar. "Y-You see...Lisa got a tick stuck on her butt and I-I'm just trying to take it off...R-Right, sis?"

"Y-YEAH! THAT'S RIGHT! Bart was just trying to help me out with this pesky little blood-sucker!" Lisa sheepishly chuckled.

When Marge looked at her son with more attention, her anger seemed to quickly change to embarrassment, but just half of it as she blushed and pointed at him.

"Was the bl-bloodsucker the one that made his crotch look like that then?" She said, pointing at his lengthy and throbbing boner.

Bart looked down and with eyes, he noticed that his boner was throbbing like crazy and struggled to free itself from his pants. Normally, Bart would be embarrassed to get an erection in front of his mother, but when he finally noticed his mother's free curves...He grew a teasing smile on his face while having his hand on his pockets.

"Yup, It sure is. But maybe we should talk about the fact that you're practically half-naked, mom?" Bart teased his mother while pointing at her.

Marge didn't know what he was saying but then she looked at her clothes, seeing how her breasts had popped out "Oh god damn it!" She said, before adjusting herself "These clothes are just too small now, I gotta buy new ones. Don't change the subject young man"

But the moment the extremely voluptuous blue-haired MILF laid her eyes on her son once again, she gasped in horror when she saw that he had stripped off his clothes and was now in his naked glory. But what got her attention was Bart's penis...It was HUGE! This mighty penis was pulsing with energy and his balls were the size of freaking tennis balls!

"Dear mother of God...That's his dick! I can't believe it! It's much bigger than Homey's!" Marge thought to herself in disbelief and amazement.

"You see this mother? This was done because of Lisa, my dear little sister ass twerking skills make me this hard every single day" Bart said as a matter of fact "I don't know if anyone would even do better than her"

If that didn't terrify Marge, the blue-haired MILF could only watch in horror as Lisa started to strip off her clothes as well and like her brother, is now naked as well. The 14-year old girl painted like a bitch as she crawled on all fours while twerking her big phat ass.

With an arrogant smile, Bart slowly walked towards his mother, who fearfully backed away from her son. Licking his lips, Bart loomed over his mother as she was backing towards the wall. "But there might be someone who could be better than Lisa...You, mommy~! I mean, look at you...Huge bouncy tits...Babelicious legs...And most of all...A gigantic mountain-sized phat ass...God, just seeing you makes me horny!"

''What the hell are you talking about Bart? You're asking me to twerk for you or something? This can't be real, Lisa would never act like such a slut" Marge felt hella surprised.

Out of nowhere, Lisa appeared behind her mother and began peeling down both Marge's thong and dress; which revealed her naked voluptuous form with her massive tits and humongous phat ass. Marge's attempt to stop her daughter was a forlorn one because Bart tightly grabbed her wrists with his hand in a very strong grip. She was horrified when Lisa started to grope her massive tits which caused her to almost let out a moan.

"Aw, I'm so jealous of you, mama!" Lisa pouted as she squeezed her mother's tits like she was trying to milk a cow. "You've always been so curvy and voluptuous...It was bad enough that you've got the BIGGEST tits...But now that you've got the most GIGANTIC and FATTEST ass?! So unfair~!"

The blue-haired woman had no idea how to react to the way they were acting, just a few seconds ago they were acting as if they wanted to keep what they were doing a secret, but now? It's like a switch was turned on in their brains and now the two were different.

"To be fair sis you weren't really that happy with having a big ass. But I suppose is only fair to be jealous, looks like you got her genes, right mom?" Bart smirked, getting closer to her.

"Y-You two are insane?"

Finished with groping her mother's tits, Lisa harshly shoved Marge towards Bart who easily caught her in a passionate heated embrace. The boy locked lips with his mother as his lips wrapped tightly around her mouth gingerly sucking her lips into his romantic yet highly erotic embrace. Marge moaned in despair as Bart continued to kiss her as he cupped her enormous bubble butt ass and groped them eagerly with his hands. Minutes of kissing his mother and groping her gigantic booty, Bart separated his lips from his mother.

Marge was completely dazed from the kiss with drool running down from her mouth while her pussy grew extremely wet from excitement. With an evil-looking smile, Bart leaned towards his mother's ears and whispered a deep command. "Shake dat ass, bitch!"

"Yeah! Let's shake our asses mom, and Bart shall decide who has the best ass in the house, the winner shall get that dick for the whole day" Lisa said, eager.

"That's right, mom! Why don't you drop the loyal wife-act already and shake that huge phat ass of yours!" Bart smirked as he saw his mother struggling to get up before he rubbed her vagina with two of his fingers. "Look at you...So wet! You really are in reality a bitch in heat! Man, ever since you've got those nice curves during the past years...Dad hasn't been able to satisfy you, right? God, that bald fatso sure is useless, huh?"

"Whatever you say, mom!" Bart shrugged as he got up and made himself seated on his chair while his mighty penis stood in its huge glory. "Now, let's shake those asses...Mom...Lisa...Just remember...Once I've decided who has the best ass will be my personal cumdumpster for the entire day. So, are both of you ready to rumble?"

"YES, BART! WE SURE ARE!" Lisa cheered as she bounced around in excitement while Marge turned her head away from her son while crossing her arms, which caused her massive breasts to look much bigger.

"Can we just get this shit over with already?!" Marge grumbled to herself impatiently, which only made Bart and Lisa chuckle at her expression.

"Wait let me put some music first"

*Cue to- Anaconda by Nick Minaj*

"Hell yeah, now this is a song about big asses, shake away ladies"

"Alright...I'll start first!" Lisa shoved her way past her mom as she stood in front of Bart with a seductive look while she slowly swung her hips.

Turning around with swift movements, Lisa started to slowly tilt her hips from side to side in a lazy swirling motion, her large asscheeks wiggling and jiggling against each other. Shaking her amazing ass, Lisa got on all fours like a bitch in heat just waiting to get mounted, began to shake her ass shamelessly for Bart's enjoyment. Her cheeks clapped and bounced against each other in a rapid-fire, openly loud assault, beating like some obscene drum. If anyone like Millhouse had ever asked her to bend over like a slut and shake her ass for them, she'd have kicked their teeth out without a second thought, but now she was happy to do it all on her own initiative just to turn her brother on, to get more of his attention and more of that amazing cock all to herself.

Her booty rocked up and down, cheeks clapping back down even as she bounced from side to side, twerking like some cheap stripper who'd just had a wad of bills stuffed in her underwear by some lecherous patron. With her big juicy ass and pure slutty enthusiasm, Lisa could have taught a master class in how to shake your ass like a cheap slut.

*Clap clap clap!*

Lisa looked back over her shoulder as Bart had his eyes on her huge ass as he stroked his huge penis with his hand. Determined to have Bart's dick for herself, she continued to shake her amazing bum towards her aroused brother.

"Holy shit Lisa! You got me so hard right now, totally worth making you believe more in yourself after that" Bart said, enthusiastically.

"Yeah, it's not like mom knows how to shake it like that, she doesn't like her ass, and I bet she is too much of a prude to do so," Lisa said, taunting her.

Despite Marge finding this situation disgusting and disturbing, she couldn't help but feel angered that her daughter mocks her. Flaring her nostrils like a raging bull, the blue-haired MILF marched beside Lisa and shoved her to the bed by bumping her incredibly wide hips.

"So, I'm a prude, huh? Well then, you little brat...Let me show you how a real woman shakes her ass!" Marge declared in a confident tone.

"Oh ho! Do you see that Lisa? We are about to see what mom's moneymaker is truly made of if only I could record that, but I just have to burn the image in my head" Bart said, drooling.

"Oh, shut your mouth, Bart!" Marge growled at her son as she turned around and lowered herself in a squat. "Now remember...This will be the first and only time you will see this massive, mountain-sized booty twerking...So you've better enjoy it!"

"Mountain sized indeed, how big is all this meat anyway? 60 inches?" Bart could feel his hands feeling grabby.

Before the 16-year old boy had the chance to touch his mother's wonderful enormous fat ass, he yelped in pain when Marge slapped him on the wrist which caused him to quickly pull his hand away in pain.

"Remember...You can see...But there will be no touching!" Marge growled towards Bart in a dark tone. "GOT THAT?!"

"Oohhh you want to tease me, mom? I like that, sure show me what you can make off" Bart smiled at how dominating she was.

Gritting her teeth in anger from her son's perverted behavior, Marge let out a tired sigh and just wanted to get this "twerking" over with.

Biting down on the bottom lip and shutting her eyes in humiliation, Marge started swaying around her body elegantly and swiftly. She was thrusting her hips into the air, shaking her extremely fat buttocks in a salaciously enticing manner that made Bart salivate like crazy. She majestically moved around, twirling her body while flaunting her incredibly large breasts and spreading out her inner thighs to reveal much of her moistening pussy without shame. Marge also demonstrated some of her physical prowess from being both a former bodybuilder and police officer by lifting up one of her legs high to the point she could easily grab onto the ankle.

Bart panted like a dog from his mother's erotic dance and stroked his huge dick much more with excitement. He was seeing firsthand how flexible his mom really is as she held up her right leg by her head, enticing him even more by spreading open her pussy with the other hand by using her fingers.

Marge slowly brought her leg down and resumed her erotic dancing by slowly twirling around her hips like a belly dancer in front of Bart with a scowl on her face. She made sure to put plenty of weight into her buttocks, letting her huge bouncy cheeks sway while keeping her arms held behind her neck while she arched her back and thrust her chest forward. Her movements were causing her massive tits to jiggle salaciously in response.

"Whoa! For someone who didn't want to do this, you're surely giving your all eh mom? You were supposed to just twerk but I might enjoy all the effort you're putting there to surpass your daughter" Bart complimented her.

"Hmph! Show off" Lisa quiped.

"Whoa! For someone who didn't want to do this, you're surely giving your all eh mom? You were supposed to just twerk but I might enjoy all the effort you're putting there to surpass your daughter" Bart complimented her.

"Hmph! Show off" Lisa quiped.

"S-Shut up!" Marge shouted at her children in embarrassment, while she continued to twerk her gigantic phat rump. "This is just a one-time thing only...So you better not get any wrong ideas about that...Am I making myself clear?!"

"Well mom it looks to me like you're trying really hard to win against Lisa, do you really want my huge cock up those cheeks that bad? Im touched"

Bart got up from his chair with a cocky smile while his dick throbbed in a very exciting way. "It looks like we have a winner...Congratulations, mom!"

"Awwww..." Lisa looked down in sadness that she lost to her mother, but quickly perked up when her brother patted her on the head.

"Don't worry, little sis! Tomorrow, I will give you a special prize for trying your best in the twerking contest!" Bart said soothingly to his younger sister as she ruffled her hair.

"Thanks, Bart! Make sure to fuck mommy like crazy~!" Lisa giggled gleefully at Bart while Marge was frozen in fear from hearing her daughter supporting Bart's decision to fuck her like some kind of breeding sow.

"Don't worry, Lisa...I've planned too!" Bart growled while hungrily staring at his mother, who gulped nervously of knowing her fate.

The boy grunts and lets out an animalistic growl as he marches towards his mother, revealing to Marge his height, and for some reason, this time, seeing her son tower over her made her pussy extremely wet. Marge was frozen in fear as she looked at her son, his cock throbbing, his presence so commanding and dominant, she couldn't help but shiver in anticipation, knowing that her womb was most definitely in danger.

Without a word, Bart grabbed both of Marge's ankles and flipped them up, the blue-haired MILF letting out a yelp of surprise as she felt her son grab and manhandle her. When Bart finally had her the way he wanted her, the blue-haired MILF was in a mating press position, her pussy presented right to her son as she laid on her back, Marge letting a small bead of sweat run down her forehead.

"Oohh Yeah mom, you may not know this but whenever I get this urge to mate, I feel such surge of energy from my body, but don't worry I will be sure to give your pussy what a dick truly is and Lisa is gonna watch us all the time so she can learn more for when I do her too. Just stop thinking too hard on this and let it flow...Marge"

"N-No, Bart...DON'T! You can't do this! This is so wrong! I'm your mother and you're my-AH!" Marge cried out as she suddenly felt Bart's massive cock shove its way deep into her pussy, the blue-haired mother's eyes crossing as she felt his dick break through her cervix and slam into the back of her womb with one, strong thrust.

Marge's eyes watered slightly as she looked down at her belly that was now bulging, her womb and pussy filled to the brim with Bart's fat cock, making the woman squirm in her position beneath her Adonis-like son.

"GOD, IT'S TOO BIG! TOO BIG! STOP, IT HURTS!" Marge moaned out as she could feel his cock throbbing inside of her. She could barely believe that Lisa was able to take this cock.

"P-Please, stop it already, Bart! I can't betray your father-aaahhhh~!" Marge groaned out as she felt Bart slowly pull his cock out of her drenched pussy, the 16-year old boy's cock pulling out until only the tip remained. "You're my bitch now, mommy!" was all her son growled out, making Marge's spine shiver as she knew at that moment, she wouldn't be walking when this was over with.

Bart then immediately slammed his cock back into his mother's pussy. Marge couldn't stop herself from howling in pleasure as she felt the wind slam out of her lungs and before long, Bart began a brutal campaign on her pussy. The teenage boy's cock hammered into Marge's womb, making her belly bulge with every powerful, driving thrust he made, making Marge wrap her arms around her son's back, holding onto him for dear life.

"O-OHH! M-MYYY! G-G-GOOODDDDD~!" Marge cried out as she finally realized what kind of a sex-beast her son truly is. No wonder he was able to make Lisa turn into a complete slut. As much as she hated to admit it, not even her Homey could give her this pleasure.

Marge could feel every thick, throbbing inch of Bart's cock, the woman's nails scratching her son's back from the feeling of the tip of his cock slamming into the back of her womb over and over again. Naturally, Marge's own mind began to empty, her eyes crossing, her enormous tits bouncing and occasionally slapping her face due to the rough fuck, but above all else, she accepted a key truth for her future… She would be unable to live without her son's mighty dick pounding into her like this every day.

With this truth finally dawning on the blue-haired MILF, she decided to fully submit to her son, her legs wrapping around his ass, locking him in so he couldn't pull out, and with a slutty smile on her face; she accepted her fate of becoming her son's cumdumpster together with her daughter.

"Oh my, looks like I can't back out now huh? Do you think dad would mind if he ends up having to raise another kid? Hahaha, whatever let's give Maggie a little sister now shall we mom?" Bart grinned, ready to cum.

"Get ready mom, while Bart hasn't cum inside me yet when I tried to drink his blast of cum I almost drowned, so you're in for a big one," Lisa said, being jealous.

"AHNN~Here it COMES! Take it, mom, take all of my pent up love for you"

Bart screamed loudly before he moved his hips a few more times before releasing thick streams of white creamy shots of his natural protein shake. Marge's eyes widened as she felt her son come inside of her, she let out choked gasps as she felt his cum bloat her womb and fill her with his thick, white essence.

"S-So much c-cummm~! I'm going to give birth to my baby boy's baby~! I fucking love it~! Thank youuuuu~!" Marge moaned out as she looked up at the ceiling with an extremely slutty smile. With his orgasm slowing and eventually stopping, Marge's belly looked completely bloated like she was already 9-months pregnant. Marge simply laid there in a slowly growing pool of cum that oozed from her pussy after Bart pulled his still erect bitch-breaker out of her pussy.

"Holy shit, Bart! I think you just broke her!" Lisa said amazed with her eyes widened from seeing her brother impregnating their mother!

"Hmmm...I believe you are right, sis! But I'm far from being done with her~!" Bart shrugged carelessly towards his sister before he turned his attention back on his mother with a mischievous smile. The siblings saw that their mother had tiredly got up on all fours, which gave Bart a perfect view of her gigantic, massive phat rump in its meaty glory.

"Now then, what to do about this ass of yours?" Bart asked himself as he walked around the floor while staring at his mother's massive mountain-sized ass once again, the 16-year old boy delivering a harsh slap to her huge fat rear once again, leaving a nice red hand mark on her ass, effectively knocking her out of her cum-coma with a small orgasm. As soon as Marge felt Bart's hand on her ass, she immediately began playing with herself, rubbing her clit in hopeless masturbation as she simply did whatever her lust-crazed mind told her to do.

"I think it's about time I claim this asshole of yours as well, mommy! You'll more than likely find it hard to walk in the morning, not that I care in the slightest," Bart said in a husky tone as he aimed his cock at her puckered asshole, Marge squealing in delight as she felt his thick rod probe at her asshole, and before anything else, Bart plunged himself inside of Marge's incredibly tight hole.

A loud, ecstatic scream came from the room that echoed through the Simpsons residence.

"Fuck..."

Lisa just stops watching and goes to stare at the window and the night starry sky, ignoring her mom's moans as she is dominated and fucked by her big-dicked son.

'If only she knew I didn't have sex with him yet and have been doing nothing but blowjobs and buttjobs just so I could get ready for the real action, well maybe next time. If he hasn't emptied himself inside her all the way'

Tonight was going to be a very messy one and god knows what else would await this situation, but Bart didn't care as he made his thick mom his bitch.