Speaking with your father may give you some insight into the situation, or not. It may be that your father has given up and just stays out of what seems like an unresolvable problem between your mother and sister. I witnessed a similar dynamic with two sisters in my family that was never resolved. It was heartbreaking, but there was nothing I could do to help the situation. They were stuck: one was a bully and the other could not stand up for herself. This went on all their lives. I believe their mother could have stopped that destructive interaction early on but never did. I'm sorry I don't have any great suggestions to solve this problem.