I slammed the door behind me and stared at my little sister, now twenty-one and home from college for the summer. "You been snooping in my room again, Emma?" I growled, already hard just from the way her tank top stretched over those big tits.
She smirked, biting her lip. "Found your stash, big brother. That growth formula you been mixing. Figured I'd try some myself." She peeled the top off, and fuck, her nipples were already darker, puffier. "But it only works on you if I get it from the source."
My cock twitched. "You want me to fuck you while that shit makes me grow?"
"Exactly." She dropped her shorts, spread her legs on my bed, and showed me her shaved cunt already glistening. "Pound me raw. Fill me up. Let's see how much bigger that dick gets inside me."
I stripped fast, my thick nine-incher already throbbing. Emma grabbed it, stroking hard. "Come on, bro.
Shove it in. I want every inch stretching my pussy while you swell."
I climbed on, no teasing, and rammed straight into her wet hole. She gasped, back arching. "Fuck yes, that's it. Deeper." Her walls squeezed me tight as I started pounding, balls slapping her ass with every thrust.
Halfway through the first minute her eyes widened. "Oh shit, it's happening. I can feel it… growing inside me." My cock was thickening, lengthening, pushing deeper with every stroke. The formula hit hard once it mixed with her juices. Another inch slid in, then two. My shaft stretched her wider, veins pulsing as it ballooned.
Emma moaned loud and filthy. "Bigger, fuck, it's splitting me open. Don't stop, brother. Grow that cock in your sister's cunt." I slammed harder, watching her pussy lips stretch around the new girth. It kept swelling, ten inches, eleven, the head battering her cervix while my balls swelled too, heavy and full.
She clawed my back. "Feel that? Your dick's a fucking monster now. Cum in me. Pump me full while it keeps growing." I grunted, hips snapping, the new length making wet squelching sounds every time I bottomed out. Her tits bounced, and she rubbed them, nipples leaking a little from whatever reaction the formula triggered in her too.
"Goddamn, Emma, your pussy's milking me harder the bigger I get." I flipped her onto all fours and drove back in. The angle let the extra length punch even deeper. She screamed into the pillow, "Yes, wreck me with that growing cock! Fill your sister's womb!"
My orgasm hit like a truck. I buried the now foot-long shaft to the hilt and unloaded, thick ropes blasting straight into her. But it didn't stop. The growth kept going mid-climax, my cock pulsing fatter, longer, forcing more cum out with every throb. Emma shook, squirting around the massive intrusion, her belly slightly bulging from the volume and size.
We collapsed, panting, my oversized dick still half-hard and twitching inside her stretched hole. She reached back, stroking the base. "Round two in the shower? I want to watch it grow while I suck it."
I grinned against her neck. "You asked for it, sis. This cock's not shrinking anytime soon."
