Bart Simpson woke up suddenly, to the weight of someone by his feet.
"Happy birthday, bart" said Lisa, holding a cake with a 16 candle planted in the middle.
Bart stretched and sat up, only slightly aware of his usual morning wood, as he took the cake and hugged his sister.
Just then, Marge walked in, also beaming.
"Happy birthday my special little guy" she said, kissing him.
"Thanks guys" said Bart.
Marge left to go take Maggie to day care, and Lisa sat down by Bart's knees.
As they had gotten older, puberty had been very generous to Lisa - she had grown a nice D cup set of tits, creamy and bubbly, to accompany her curvy ass.
She had swapped her orange dresses for tight jeans and open topped shirts, to show off her figure as best she could.
She usually slept in a bra and some very short shorts, which only furthered the illustrations of her huge ass.
She was wearing this attire right now, and Bart noticed that his erection was at breaking point, protruding like a arm from his crotch.
Lisa was talking animatedly, her tits jiggling as he did, about Jessica Lovejoy - the girl who was seen giving principle Skinner a blowjob to raise her grade.
Lisa suddenly stopped, realising Bart had an erection.
She gasped. It was straining against the cover, and looked a good 10 inches. She had seen dicks before - having sucked off Rodd and Todd next door just a few days ago - but nothing this huge.
Bart noticed and started breathing more heavily.
They locked eyes - pools of lust, both wanting it and both too scared to make the move, neither wanting to do something wrong.
Lisa got up suddenly, and ran to the door, giving Bart a great view - the shorts had ridden up her arse to almost the point of non-existence.
She looked left and right, then, satisfied, closed and locked the door, before turning to Bart.
"Get rid of that fucking duvet" she growled passionately.
Bart complied, throwing off his boxers as well, revealing his nine inch dick, as well as his toned abs.
Lisa was already walking to him as she undid her bra - it fell to the floor, exposing her tits.
She jumped onto him, bending down as the two siblings begun making out passionately, Bart an able to keep his hands off her tits and arse, off the places he'd been spying on for years. Lisa, too, unable to stop jerking his dick, until he lifted her up into the air and layer her down, before crawling down to her pussy, removing her shorts.
She moaned as he begun kissing her inner thighs, tracing her entrance, teasing her.
Her hips bucking, trying to find his tongue, before she whispered "please".
Bart dove in, lapping up her flowing juices and licking her clit, expertly flicking her clit, before licking her folds and probing her entrance with his tongue.
She obviously paid attention to hygiene - she tasted of strawberries, while he obviously was experienced - Lisa came after two minutes into Bart's smiling face.
She took a moment to compose herself.
"Get up here Bart, it's time for your present, birthday boy."
Bart shuffled up and put his head on his pillow, looking at Lisa, who was now jerking him off, before she put the tip in her mouth.
Bart began moaning as Lisa moved her head up and down, her skilled tongue rolling over his tip, licking the side, and rolling back to the tip, all the while as she moved up and down quickly.
Bart held her head as she blew him, pausing only to lick his balls, and soon he had an urge to cum.
Lisa felt it too, so she took it out and laid down next to him, as he turned to face her.
Sideways, and still kissing, he pushed his dick into her , and begin thrusting, holding her leg and arse as they made out.
Her pussy was amazing - warm and wet, perfectly tight, squeezing his cock just enough.
Bart got on top and started ploughing her, sucking her tits while she moaned.
Then, just when he was about to cum, he pulled it out, flipped her over, and resumed in doggy.
Bart had recently bought a mirror, and now he gazed into it, seeing himself fucking Lisa, her tits jiggling as his balls slapped her legs.
They fucked for another fifteen minutes, before Lisa came around his dick, screaming into his kiss.
Bart pulled out, put his dick in her mouth, and came hard, shooting pump after pump into his sisters throat, until he collapsed next to her. She swallowed it all, and then went down to clean up his dick with her mouth, before they kissed again.
"Happy birthday" she managed, after they both came down from their orgasms.
"Wanna share a shower to get cleaned up?" asked Bart.
She nodded, and Bart followed that curvy ass into the shower, already hard again.
Happy Birthday Bart.