What can I do about my sister?

Tell her that if she touches you or one of the other family members again, you will call the cops. If she becomes abusive, then follow through. She will see how she is the "pussy" in jail. Maybe it will scare the sh*t out of her and she will knock it off. I have been in a similar situation as you. It sucks to have to call the police on a family memeber, but she needs to listen to someone and she apparently doesn't feel the need to listen to her parents. Fortunately she HAS to listen to the authorities.