Sierra couldn't really give much of a damn about her mom, but the thought of Blair in distress sickened her. She ran straight for Blair's room, not caring at all that she was still naked and looked like she'd been made up for a werewolf transformation scene in a movie. The important thing was to make sure Blair was safe.
When she flung open Blair's door, Sierra was greeted by the sight of a naked Blair sprawled out on her bed, a horrified look on her face. Blair's exposed body was coated in shaggy red hair, just like Sierra's. It had the same semi-mane, the same hairy pits and arms, and the same furry pussy bush that grew up to the bellybutton and down the legs. Blair's pubes and fingers were sticky with her juices; she had obviously been masturbating. Sierra was struck by how buff Blair looked now. The 16-year-old's formerly slim body now rippled with toned, hard muscle, including a fine set of six-pack abs. Her nails had sharpened slightly like Sierra's. Oddly, instead of pointed canines, a pair of what seemed to be the beginnings of tusks poked out of Blair's mouth. Two bony buds crowned the sides of Blair's forehead. Proto-horns, perhaps?
Sierra gawked. Her little sister's transformation was dumbfounding... and more than a little arousing. Sierra tried to push the distracting horny thoughts from her mind to focus on comforting her frightened sister.
"Blair?!" Sierra gasped. "Are you alright?"
Blair ogled Sierra's new body a mixture of fear and lust in her eyes. "I... I had this weird dream about... werewolves, I think. It was so hot. When I woke up, I was REALLY horny, so I stripped and fucked myself for a few minutes, and... THIS happened when I came!" She gestured at her altered body.
Sierra licked her lips. "D-don't be afraid, Blair. I didn't mean it for you, but it's harmless."
Blair's eyes bugged out beneath their bushy eyebrows. "YOU, did this?! HOW?!! WHY?!!"
"I bought this potion in a magical supply store. Don't both saying there's no such thing as magic; just LOOK at us. Anyway, the stuff's called Beast Juice, and I took it to undo that laser treatment Mom forced on us. Then I snuck some in the coffee to give Mom a little hairy payback. You had some of the coffee, so you changed, too. Don't worry, you'd have to take more Beast Juice to change any further. Sorry about this."
"Hey, it was just an accident, Sierra," Blair smiled awkwardly. "No worries. Oh, GOD, I feel so horny right now!" She gazed into Sierra's eyes the expression mad with animalistic lust.
"Yeah, I feel it, too," nodded Sierra. "That's a side effect. Just count to ten or something." She hoped the two of them could control their instincts... or did she?