Hi Carbon12,
welcome to PC. I'm so sorry you suffered abuse and your sister is still under their threat. How old is your sister? You could report their abuse to childrens services. If she's of legal age, would it be possible for her to live with you or report them and she could be moved to a home for the disabled? Due to her disability, she's more closed off than the normal person and may live there long into adulthood. You're doing the right thing coming out in the open about this and the next brave step is doing something.