Follow Adam into the changing room and give him a little help.

Shooting a glance to Katie, Sarah saw that the clerk was folding clothes at the edge of the changing area. Stealthily, she slipped into the changing booth with her brother and was not really surprised to find him kneading his sizeable tits and rubbing his pussy. He was looking into the mirror, his face flushed with arousal, mouth open in the O of pleasure she had learned to love so much. He was wearing the black lace ensemble with bra and matching panties Sarah had picked out for him - she was surprised he had been able to put it on without her help. Had he been watching her change some time in the past? No matter, she had come for other things.

Adam noticed her stepping into the booth, but before he could react, she was behind him, pressing her own modest breasts into his back and her growing hard-on into his buttcrack, only two layers of fabric between her and Adam's sweltering honeypot. He moaned and started slowly grinding his slightly enlarged ass up and down on her quickly stiffening pole. Sarah grabbed his tits from behind and started massaging them through the sexy piece of lingerie that held them. Even through the cups, she could feel his stiff nipples poking into her palms. Sarah leaned forward and whispered into his ear.

"I noticed your cute butt has gotten bigger. You know what I think? My cum's having an effect on you."

Adam's only answer were deep, aroused breaths and soft moans. Didn't his voice sound higher than in the morning? Maybe Sarah was imagining things.

"Every time I fuck you, you get girlier, sexier, sluttier. And you know what? I'll keep fucking you until there's nothing left of the old Adam, only Ada, my little slutboy. Would you like that?"

The idea of becoming his sister's perfect little bitch lit a fire inside of Adam. Every day, he would get railed by that magnificent tool between Sarah's and be pumped full of her delicious sperm. Every day, his tight cunt would be plowed and his tits would be played with. Gone was his wish for a bigger dick - his sister's cock was the only thing on his mind. He leaned forwards until his cheek touched the glass of the mirror, still grinding his ass against his sisters dick stretching the seams of her jeans, both hands pressed against the glass. Adam reached down, shifted his panties aside and spread his pussy lips open. Looking back with a half-lidded, lust-filled stare, he pleaded:

"C'mon, please. Stick it in, make me your little slut!"

He didn't have to ask twice. Sarah took a step back, Adam mewling as his butt lost contact with the magic tool that would make him scream so soon, following it as far as he could. Dropping her pants, she freed her one-eyed monster and began teasing his labia with the head, always moving back a bit when Adam tried to push his crotch back to push Sarah into him.

"Ah ah ah, I want you to beg for it," she breathed.

Adam did not hesitate. "Oh please, please, please stretch me out. Break my pussy. Creampie me!"

"Ask and ye shall receive," Sarah grinned. Outside the changing booth, she could hear a soft gasp. Apparently, someone in the shop was a dirty little voyeur! Her grin became wider, only to be replaced by an expression of pure bliss when she slowly pushed her cock into her brother's waiting cunt.

"Ooooongh..." came her brothers eloquent response to being spread open by her throbbing member. Grabbing his hips, she slid out halfway only to ram her slick tool all the way in, prompting a low, lustful moan from her little slut.

"Ungh, yes! You're... so... fucking... hot!," Sarah exclaimed, each word accompanied with a powerful thrust, her heavy balls smacking into his thighs. From outside, she heard labored breathing and lewd, wet sounds. It seemed they weren't the only ones enjoying themselves. To give their audience a better show, Sarah started gyrating her hips, varying the angle of her entry. Adam definitely enjoyed it, judging from how his eyes unfocused and his moans rose in volume. He took one of the hands he used to braced himself against the mirror and started rubbing the swollen nub peeking out from it's hood. The pleasure intensified, but he wasn't quite there yet. He needed more!

Luckily, Sarah was intent on giving him just that. Noticing his rising lust, she started jackhammering away at his stretched cunt, making Adam squeal in pleasure. A muffled moan came from outside the booth while the squelching sounds intensified. Having a little watcher all of her own made her dick tingle and Sarah couldn't keep from groaning lustfully.

"Take it, Ada! Take my hot, throbbing cock!," she moaned, leaning forwards until her breasts touched Adam's back. His tits were smooshed against the mirror, but she wriggled her hands under them to squeeze and fondle the pliant flesh. The sudden attention to his sensitive mounds drove Adam over the edge. His pussy clamped down on Sarah's veiny rod, twitching and milking it for all it was worth. His leg-muscles shook, losing all strength, and moaning loudly, he slid down the mirror, leaving behind a trail of drool, until he sat on his lower legs. Sarah's dick slid out of him with a pop, but in his orgasmic bliss, he did not notice.

Turning around, Sarah pointed her throbbing cock towards the door of the booth, Katies shocked blue eye looking at her through the crack, her hand frozen deep in her honeypot.

"Hey honey, want some?," Sarah asked, shaking her schlong invitingly.