The obvious solution

Given latitude, the magic stone powering the wish chain would always look for the least disruptive solution to grant a wish. Least disruptive from the perspective of the stone that is, not necessarily from the perspective of the former Jon Madison, who suddenly became the big sister Zoe used to worship, at least until they both got to the same high school and Zoe switched to following the Queen Bee of the school Sarah McMullen. Zoe knew if she played her cards right, after Sarah graduated, then she could be the new alpha. Her sister was nice enough, and certainly had the look, but she was just too nice and too meek to rule the school. The memories of this new reality quickly etched themselves into the brain of the blonde, buxom Joan Madison. Joan glanced over at the stone sitting on her makeup table. She thought she saw it flash a little bit, but shook her head to clear the afterimage. It wouldn't have flashed because she certainly hadn't made a wish- not since she'd foolishly let an offhand comment to her friend Karyn create her current platinum, top heavy look. She must have been imagining things.
Joan was about to pick up the rock to see if it was warm but got distracted by the ringing of her phone.