What if your lil sister humps things she already started her period she thinks she going to get pregnant can she?

Your sister has a bad habit of doing that. It's actually a way of masterbating. You should talk to your mom about this so it doesn't get out of hand, especially if she is doing this in public. She can not get pregnant doing this. She really needs to talk with an adult you can trust like a school counselor if your mother's not available. A family doctor.