I would confront the girl on it or talk to your sister, if she's reasonable. That's terrible someone stole your makeup.
I learned my lessons well about theft in 1st grade. I would take barrettes out of my hair and place them in my bookbag. A girl would steal them from me. When I would go complain to the teacher and pretty much freakout, the girl would "find" them again. I don't care if she was like 6 or 7, what a bitch. She was supposed to be my friend, too.