Wow, that's one hell of a situation. I'm really sorry for you. That's the thing about family problems, they're all distinctly unique (for the most part) and there's never a clear cut answer of what to do. With your dad continuously complaining, harassing and arguably manipulating your sister, you should probably distance yourself from him. It appears this has been going on for some time so hoping he'll turn around doesn't sound quite promising. The best thing you can do is focus on yourself, I know that's not the greatest, most eye-opening advice to give, but you sound young and the last thing you need to do is get burned out by this. Yeah, it's not easy, but negative relationships tend to act like whirlpools, they'll suck you in if you let them. Better to work on yourself and be there for your mother. Best of luck, I know it's not easy.