"Mmm... this is a pretty amazing rock, Jon. We could have some real fun with this. In fact, I wish we were at my house." In another flash, their surroundings changed. They were in Katie's room, a pink place, though not nauseatingly so. In place of her bed, however, was a bunk bed. Most of the items around her room were larger now, as if they had to accommodate two people. Jon had a very bad feeling about being in this room.
A somewhat familiar voice called out from the doorway, "When did you two get home?" Katie's mom stood in the door, smiling at them. "Oh well, I'm glad you're here. Your sister is a real handful. Can you take turns watching her?"
Jon was speechless. Katie answered happily, and turned to Jon after their mom left. "This is amazing! You're my new sister!" She pulled him into a tight hug before he could react, putting the rock down on the bed. He was preoccupied by another deep kiss, and by a wandering hand on his breasts.
"Eww! Mom! They're kissing again!" Katie gasped and pulled away from him, looking towards the door. A glimpse of Katie's little sister... mine now, too... was visible before she ran off.
"Sammy!" Katie ran off, and Jon picked up the rock. Could he really do anything with it? There didn't seem to be any clauses he could add to Katie's wishes to make them bearable. A time limit may not work. Another wish to be able to transform himself or have an item to would probably not be able to change him from what he was, but he could change Katie...