I read your post on Kaiaa's thread "What's Bothering You?".
I'm so sorry, I know where you're coming from. Maybe you should talk to your mom about it or an adult you trust, whether it be in your family or at school. From my point of view, your sister is doing nothing wrong and your father is practicing bad parenting. Maybe when it happens again, step in (but dont put yourself in a position where you yourself will get hurt) and ask her what's wrong before he gets to her. Reverse roles and make yourself a role model, let him see what makes your sister perform better other than harming her. Please take care of yourself and look out for our sister. I'm here if you ever need to talk okay?