Depends on what they did, their ages (and yours), and how much trouble you'd be in afterward.
are you stronger than her?
Incestuous suggestions; the ultimate tactic in creepy revenge.
Note: could backfire in the worst possible way.

If you're not vastly stronger than a girl one year younger than you, then something is really very wrong.I'd like to think so lol. I never really had a who's stronger contest with her though. I would hope I am stronger than her though lol.
I'd try and become her friend some how.