She get's a little slap-happy.

"Alright, slutboy. Get down on all fours, ass to me. And no moving," Sarah ordered. As Adam complied eagerly, she noticed that not only his lips had changed: His boyish hips and flat ass were gone, replaced by a juicy butt atop thicker thighs than a guy should ever have. Sarah bit her lower lip. The sight of Adam's drooling pussy and his twitching asshole nestled in between those jiggling orbs helped considerably in stiffening her cocks again.

Taking each in one hand, she began slapping Adam's buttcheeks with them, reveling in the waves this sent through the pumped up flesh. She would never have thought her brother's ass would ever drive her wild like this, but neither did she ever imagine having a cock, let alone two. The thick meat colliding with his behind sent waves of pleasure up Adam's spine, making him shudder and moan in bliss. His pussy dripped its juices onto the hardwood floor, worked its lips against the air, begged to be filled. His sphincter relaxed, gaping slightly, waiting for his sister's cocks.

After having gotten her fill of watching her brother's butt jiggle, Sarah lined up both her swollen dicks with Adam's holes and rammed them in without further ado. A wailing moan rose from her brother's throat. His voice could now only be called sultry, few traces of his old tenor remained. Adam felt fuller than ever before. Both his holes were stuffed with the slick, throbbing poles jutting from his sister's crotch, the pleasure from this twin penetration making him go crosseyed. It was incredible; having his pussy and ass stretched to the limit at the same time almost drove him over the edge right then and there.

But Sarah had other plans - this was supposed to be a punishment after all. She raised one hand and slapped Adam's plump rump hard, leaving a red outline of her hand behind, causing a surprised yelp to escape Adam. The pain of the slap traveled right into his clit, making it throb with a mixture of pain and pleasure. His orgasm seemed now a bit further away, but that did nothing to lower the loudness or intensity of his moans and gasps.

Sarah herself was not exactly quiet. Feeling the tightness of Adam's asshole and the velvety slickness of his pussy simultaneously made her eyes unfocused and her tongue loll out of her mouth. She started bucking her hips, unable to deal with the pleasure from her twinheaded crotch. She kept slapping Adam's jiggling buttcheeks with both hands, alternating between left and right. The stinging pain made Adam squeal with every hit and mingled with the lust that had long since overwhelmed him. Every hit made his abdominal muscles clench, squeezing Sarah's pricks tightly and forcing deep moans from his sister's throat.

His orgasm kept building and was delayed again and again by the slaps to his posterior, but Sarah's hips started moving more and more with each stroke until she was hammering away at Adam's holes, each thrust accompanied by the sound of groin slapping into ass and loud squelching noises from Adam's ass and pussy. Sarah had stopped slapping her brother with her hands, instead hanging onto Adam's hips for leverage. He had fallen into a rhythm that perfectly matched her own and made sure that she bottomed out at the end of every single stroke, making his lower body quiver with lust and his ass and thighs jiggle in waves.

Their moans rose to a crescendo together and with a particularly vigorous thrust, Sarah spilled her seed into Adam's bowels and womb, pumping her hips through their orgasm. The delayed orgasm hit Adam like a freight train and he came harder than he would have ever though possible. His arms gave out and he collapsed to the floor, his tits squished into the hardwood by every hit slamming into him from behind, his nipples dragging over the floor, adding to his orgasmic bliss. Only Sarah's stiff cocks held his behind up. Adam's mind went blank, unable to keep up with the lust flooding through him.

Sarah did not let up while there was still cum inside her balls, every thrust spilling another spurt into her brother. Her enlarged bust jiggled with the pounding she gave her brother, the reminder of her new size only enhancing the feeling of her orgasm.

Spent, she fell backwards on her ass, letting Adam collapse on the floor, breathing heavily. Her cocks slowly softened, slick with their combined juices.