my sister watches me

I don't know what os more ridiculous, this story or your username.

Regardless, you stated you do lock your doors, so she must be pretty crafty in opening locks. Perhaps put the lock on the door facing upward at the top of the door she cannot reach. She is also twelve, as you stated, so you and her must be living with your parents, correct? Inform you parents so they can tell her it is wrong, and if she continues to do so or continues to try, tell your parents again so she can get punished and learn it is not a good thing to do.