Sarah thought for a moment...she hadn't actually wished to be Jon's little sister...all she needed to do was make another wish to fix it. Wish that there was some other reason he didn't think it was odd for her to be going through his things...
The stone was in his pocket, and he thought she was his little sister...she already knew that everyone who didn't hear the wish was unaware anything was different. It gave her a chance...she could sneak in and keep trying until she got it back...
