I found a nude pictures of my sister in her laptop?!?

If your father beats her, that's as bad as what she's already doing. You need help from another member of your family that you trust who will be there when you tell your dad and insists that he not physically abuse her. She needs counseling, and if your father is beating his kids, he is breaking the law, and its no wonder your sister is acting out in this way. He needs to act like a human, a parent, and a real man, and get your family the help it needs instead of breaking the law himself and adding to your sisters problems. Find someone who can be there that you trust when you tell him, even if it's the police.