Just a small peck on the lips. That's how it started anyway, Mabel went in for a kiss, the kind she would give on Dipper's cheek but instead on the lips. As she tried to pull away Dipper lunged at her, kissing her hard and deep, very un-brotherly of him. Dipper was terrified when he pulled back and Mabel was stunned, just staring at him wide-eyed. However he quickly learned she was harboring some of the same feelings. Ever since their secret romance had gone down a path Dipper hadn't expected. The late nights of sneaking into each other's room, cuddling, making out loudly in the dead of night, Dipper was especially surprised with how fast Mabel had pushed the physicality of their romance.
After only a week or so she couldn't keep her hands off him, constantly stroking his hair, nipping his earlobes, biting his neck, stroking his chest and stomach whenever they were alone, and even stealing some sensual fondling of his shoulders and butt when people were around, always to Dipper's embarrassed discord at the possibility of getting caught.
A bit of time longer, and she had gotten bold enough to start going for both his and her erogenous zones, rubbing the insides of his thighs, running her fingernails along his chest, making sure to dig into him as she presses her body into his. Taking his hand and placing it on her stomach or chest. This quickly devolved into heavy petting outside of undergarments and dry humping when Mabel's teasing had aroused the poor boy.
Eventually They let their degeneracy come into full force. Mabel, once again being the aggressor, actually plunged her hand into Dipper's boxers during a session of hot and heavy clothed groping. The initial skin on skin contact had stifled the resistance that Dipper wanted to throw up when Mabel grabbed ahold of her shaft and gave a few shakes.
The next time he wanted that bare feeling again, and she took it a step farther, putting her mouth to work. Dipper once again reasoned with himself that it wasn't so bad to go another step in his inappropriate relationship with Mabel. His buzzing mind of justifications was hazed over by pleasure when Mabel's wet mouth melded onto his member.
Just a few more mouth hugs later and dipper was too excited to go further. Finally being the one to push for more, Mabel happily obliged, laying back on her bed and spread her legs wide open. Her hand automatically moving down between her legs, spreading herself open for her brother. The site was so erotic for the teenager he couldn't contain himself, the thought of Mabel so openly exposing herself, welcoming Dipper in made him go over the edge in a frenzy of teenage lust
He took her virginity that night, She was barely able to stifle her moans and screams low enough for their parents to not hear her deflowering. But she became even more addicted to the feeling of being filled by her brother, she craved it daily. Of course they couldn't partake in the carnal delights constantly, so when they got the chance they savored it. However, Mabel was getting impatient. More willing to risk exposure to get her fix.
Now they were sitting in the back seat of their parent's car, heading up to Gravity Falls for a summer visit as is the usual. Its a long trip and they had left early. Mabel and Dipper are in the back seat, sleepily slumped together, Dipper laying against the door and Mabel laying on him, both half-asleep. A dark thick blanket is draped over them, leaving only their heads exposed to the cool morning air. Dipper is conscious but keeps his eyes closed, trying to go back to sleep. He is having trouble though, as Mabel is laying against him, her head on his chest. one hand resting on his stomach and the other on his thigh. Mabel stirs, her hand sliding down between her legs, her thumb scraping the crotch of Dipper's shorts.
He can feel the pressure of her thumb pressing against him and he wakes up slightly. He cracks open his eyes and looks down at her, Mabel is Smiling wide into him. He doesn't want to wake her. He resettles his head, trying to ignore her hand. She stirs again, her hand rotating between his legs, her palm them plants firmly on his crotch. Dipper nearly jumps out of his seat. his eyes burst open and he looks down at Mabel to see her one eye looking up at him, her smile even wider now. She puckers her lips and lets out a barely audible "Shh".
Dipper looks up front to his parents, they haven't noticed the twins were awake, still just talking amongst themselves. He closes one eye and keep the other only cracked, looking back down at Mabel. She is biting her lip now, trying to not laugh. Mabel's hand is grinding up and down on Dipper's hardening tool. Her other hand starts to slide down as well, slipping under his shirt and lightly dragging her nails up his abdomen until it comes to rest on his chest, slightly digging into his flesh as her hand on his crotch begins to subtly fumble around for his zipper.
Upon finding it, she quickly and quietly unzip him, unbuttoning his shorts and sliding that hand inside, on top of his boxers. She can feel his erection pressing against her hand. Mabel puts a finger to each side of it, scraping the outline, eliciting muted moans from Dipper. Mabel gives a teasing kiss into Dipper's chest. Her fingers close together and lightly move up and down. Dipper swallows hard and looks back down at Mabel. She is asking permission with her eyes, raising and eyebrow. Dipper only nods, unable to refuse her advances.
Her smile fades a bit in favor of concentration as she pushes her palm against him, wrapping her fingers around his shaft through the fabric of his boxers. Mabel gently, slowly begins to slide her hand up and down, scratching at his head whenever she moved near it. Dipper breath kicked up, he was audibly breathing heavy and faster. He sneaked a quick peek at the front seat, but her parents hadn't noticed yet. He takes in a slow deep breath, trying to calm his pounding heart. His moment of zen is cut short when Mabel's fingers slip under his waistband.
Her hand is cold and makes Dipper jerk in his seat. Mabel grips him hard, letting his heat warm her up. Holding her hand still she starts to rub her thumb up and down his length, teasing the tip. Her thumb presses down on his head, and she feels something sticky. She dabs it up with her finger and pulls her hand out of her brother's pants. Mabel looks up at dipper, giving him her best bedroom eyes before her hand emerges the top of the blanket. Her finger is wet and shiny. Dipper on registers it for a moment before Mabel licks her finger. The hand falls below the blanket as fast as it had surfaces and she winks at her brother with a silent giggle.
Dipper grits his teeth feeling his cock throb with lust over his sister's teasing. He was getting to that point where his mind was too fogged over with lust to care about anything else. He knew his parents were right there and they could be found out at any time, but Mabel's teasing was going too far, he wanted nothing more than to throw her down onto her back and fuck her right there in the back seat of the car.
Dipper knows he can't let on that the are doing anything, though. the last thing he wants is for his parents to catch Mabel jerking him off less than a foot behind them. He looks down at Mabel with a stern expression, trying to convey his message to just get on with it already. She seems to understand as she readjusts her grip on him and starts to move her hand up and down, letting their skin glide alongside one another.
Dipper lets out a sigh as she finally starts relieving his tension, she gets greedy though, she starts jerking him faster, cause a visible shift in the blanket every time she goes down to his base and back up. Dipper loves the feeling, but knows she should be going slower, so as to not catch the eyes of their parents if the happened to look back.
Dipper lets out a gasp as Mabel stops stroking for a moment to fondle is head, massaging and teasing his glans.
"Dipper?" His mom asks, looking into the mirror.
His eyes closed on instinct. Dipper's heart raced, and he could feel Mabel's start to pound next to him.
'What should I do?' Dipper asked, fear and anxiety pulsing in him. Here he was, his dick out and in his sister's hand. Did they see them? He decided to keep his eyes clamped shut and pretend like he was still asleep. There was a silence for a moment.
"Hmm." his mom said before turning back to their dad and continuing whatever it was they were talking about. Focused on them he heard them start to talk about plans for college, something that was still a couple years off but they had talked about extensively.
Dipper felt Mabel's hand grip him under his head and begin to stroke with renewed vigor, gripping him firmer than before, like she was really trying to milk him, it was almost painful the way she was ringing him. Almost. Dipper cracked an eye and looked down at mabel who had the biggest most thrilled smile on her face. Her eye was open and she will looking right at him. She was biting her lip and she twisted her hand around during the strokes, trying to make him cum.
Mabel had gotten better at this, and though a bit of practice she had figured out what Dipper likes and what got him off quickest. Mabel took her free hand and grabbed Dipper's wrist. She guided it between her and the seat, down between her legs. Dipper could feel the sheer heat radiating off of her, the back of his hand touching a wet spot on the cushion. She hadn't worn panties, the minx. 'She probably planned this from the beginning.' Dipper thought to himself. He pressed up and felt her, she was practically drenched, leaking even.
His thin fingers quickly slid inside of her, causing her to clench up and let out a faint gasp of pleasure. His thumb quickly moved up and found her joy buzzer, pushing the skin back and pressing down on it hard before rolling his thumb around, just the way he knew drove Mabel crazy.
She quickly quickened her pace on him, Trying to make him cum first. It was always a competition with Mabel. Luckily for Dipper, however, he never lost this contest. Between the two of them, Mabel was the one with a hair trigger. So as she frantically began beating him off, sending him closer and closer to his edge. He dug into her with his thumb, being rough with her most sensitive area.
Naturally, Mabel caved first, her soft stifled squeals of orgasm went right into Dipper's chest, unheard by the parents. Her shudders were also covered by the blanket thankfully. That didn't stop Mabel from finishing her part of the race, though. She grabbed Dipper base with one hand and his head with the other, quickly stroking her top hand up and down, rubbing her fingers harshly over his engorged member.
He was on the edge anyway and had already won the little competition so he let himself go. He lurched his hips forward, as much as he could while sitting down and felt his orgasm wash over him, he could feel his cum rising up his shaft until it came out, right onto Mabel's hands.
As Mabel felt the hot goo start to flow she stopped tugging, just firmly gripping his base as he slowly started to soften. Mabel gave him a few more squeezes to get the last of his seed out and collect it in and on her hands before finally pulling away.
Dipper quickly put himself back together, putting his pants back on before Mabel made a big display of pretending to wake up. She quickly reached down to the bag of two day old Mcdonald's trash and fished out a few napkins to clean herself up with. Dipper followed suite and pretended to wake up for the first time.
The rest of the drive was pretty quiet, stopped at a few gas stations for food, gas and bathroom breaks. At the first stop Dipper had pulled her aside and admonished her for recklessly risky behavior. She curtly told him they didn't get caught and he should relax, flashing him a smile. He still didn't like the idea of doing things like this, but couldn't argue that it wasn't exciting.
He told her to just be patient and they would have their chance to be together soon enough, but for some reason he didn't think she got the message.