I would go into her room when she's there and quietly take your things back a few times. I would treat her "gently" as if she has a problem and try to freak her out that way Feel "sorry" for her that she has to resort to taking her little sister's things, can't adjust to her own age/situation. Maybe next time you get/buy something you like and think she would, get her one too and give it to her as a gift and be all "caring" when you give it to her. When she calls you names, respond with vague, kind, responses, "That's okay, everything will be okay, I know you can't help it, I'm here for you".