Just let your sister know that this is not over, and you have a very long memory. Someday, she will need a favor from you, very badly.  You can either laugh in her face, or after much torturous conscience wrestling be magnanimous and come through at the last possible minute. By the way, where were your parents in all of this? Were there no lessons on respecting other people's property?

There are many, many opportunities. For example you simply not making here aware of something she would really like to know. Unless someone specifically asks you to relay the information to her, "Oh my, did you really wish you had known abou:t (Mary Ann breaking up with her boyfriend? The concert ticket that was available at the last minute? That you left your phone in the restaurant? That the date for the surprise party had been changed?) Even with Face book and texting, there are gaps in communication because someone assumes that surely someone must have told you.  A friend of hers stops by when she is gone, "I have no idea where she is or when she will be back" Look guilty and uneasy, like you know, but she made you promise not to tell, so when the two of them talk, her friend will be suspicious and interrogate her.  Teenage girls are all insecure and paranoid.  You won't have to do much at all to f*** with her. Teenage girls LOVE drama, so they will be crawling over themselves to talk to you to get dirt.  Just let on to them like the two of you are really close.   Your sister will deny it, and be furious with you, but you won't really be betraying any confidences, just playing her like a violin. Just watch for your opportunities, Alex. Play it cool, play it smart, and she won't be able to concentrate enough to get through math in summer school. Anyone immature and disrespectful enough to move someone's property without permission has enough issues to hang themselves given enough rope.