The perfect victim was, in fact, knocking on the bathroom door. "Can you finish up with your period or whatever?! Some of us need to shower!"
A typical sort of animosity had always existed between Mary and her brother. Daniel was... well, he was a raging dick, there wasn't exactly a nice way to spin it. Your typical jock asshole, treating people like garbage, and quite insecure about his masculinity. If anybody needed a few alterations, it was him. Ohhh, she'd like to see Daniel pursue that NFL recruitment he was obsessed with after this.
Silently - mostly because she hadn't quite figured out vocal chords yet - Mary sloshed over to unlock the door. "Fuckin' finally! What took you so-" He stopped short in his sentence as Mary opened the door, a smirk upon her pink slime face as he eyed her inhuman form up and down. Before he could say another word, a goopy hand grabbed him by the face, muffling the boy's yells as he was dragged inside.
Locking the door behind them, she was quick to get to work on her terrified brother. He couldn't scream as she kept a hand over his mouth, her arm extending so that she could keep it there as she stood up and pressed a foot down onto his chest. His clothes quickly dissolved as a bit of her slimy form oozed through his pores, instantly hitting him with a powerfl dose of the vial's alchemical power. The thrashing calmed down shortly thereafter, replaced by convulsions as his body began to change.
That beard he'd been so proud of retreated back into his face, and at the same time, the buzz-cut blonde hair started to grow out, quickly spilling into a full-on shoulder-length mane of gold, before quickly turning a pink color. His face and shoulders quickly drew in, losing their broadness in exchange for a certain delicate nature, complete with eyes that softened as their irsies became the same color as Mary's gooey form. His lips plumped up, and soon something else was plumping - pecs were rounding and growing into a juicy pair of breasts under her foot.
Mary giggled as she watched Daniel's manhood begin to shrink down, vanishing within moments and leaving behind a growing slit. Dainty hands were now running through his - no, her hair, muffled screams becoming muffled moans as his hips flared out, toned butt becoming a perfect heart shape pressed into the cool tile. She looked down, expecting to find lissome legs and instead spotting something entirely un-leg-like: a single limb, Danielle's legs having fused into a long tail that tapered at the end.
Pink and purple scales appeared up and down its length, taking on a distinctly snake-like pattern that was matched by her pink eyes becoming slitted, a forked tongue sticking out in the slime over her mouth. Mary assumed her now-sister was becoming a naga... until she noticed that her ears were growing out, not to a point, but turning pink and looking more and more like some sort of... fin. Fins - yes, she was getting fins now on her forearms, at her hips, even a dorsal fin running the whole thirty-foot length of her new tail, out to the tip at which emerged a broad, powerful tailfin as wide as the former Daniel had been tall.
She finally pulled away from Danielle, letting the new woman's moans fill the room as her breasts gained scales, surging out to beachball sizes. It seemed like it hadn't just been one vial's effects that hit her sibling - the contact had caused multiple effects that combined to create this... sea serpment, she supposed was a good turn for it. A lusty sea serpent - her former brother rose up on her tail, rising to slither twoards the slime girl with desire raging in her eyes. "Sssssissssss," she moaned, her voice almost angelic in tone, "Pleasssse... I need... releassssssssse..."
The gleeful potential of this situation drove Mary to quickly form a set of apropriate shapes within herself to be able to talk. "Why should I?" The slimy transformation goddess asked, hands on her wide hips. "You've always been... such an asshole to me..."
"I'm sssssssssorry!" she whined, practically on the verge of tears as she played at her massive chest. It was becoming apparent that something in the mix had given her throbbing libido, a desperation for sex in her inhumanly gorgeous new form. "I'll do anything... pleassssssssse jussssssst... make love to me, right here... I need it sssssssso bad...“