Your sister Jenny had eaten a red pill. But she thought that it was some candy. Yes it was but it was stronger, much stronger than what she could ever handle.
"Hmm... what was that?" said your sister. "It tastes weird, it must be a new flavor".
Interestingly, there was no effect to show that anything had happened to your little sister.
But wait wasn't that scales on her arm?
Suddenly, your sisters shirt and pants began stretching out, she started screaming as she saw the scales spreading out from the arm to quickly cover her body. Her body was being extended, and she felt a stretching sensation above her butt. She rushed upstairs as she slowly grew, to her full-length mirror to get a better impression of what was happening to her. When she arrived, her skin was definitely a vivid, olive green, except for a patch under her chin which was turning a bright yellow.
Her tongue began flicking out. She was becoming both taller and slightly more muscular. Her limbs weren't fusing or diminishing in any way, but her hands and feet were changing to have sharp, taloned claws and a digitigrade stance. Also, she noticed a very long, powerful tail growing out over her rump. By now, with her face pushing out into a flat, stretched humanoid mouth. And finally she grew 2 horns on her head.
You crept up to her room and ask her if she's ok. But you come face to face with a 7'10" dragoness with FF cup breasts.