What can I do about my father's physical abuse?

This story makes absolutely no sense. Why did he start yellling at your sister? What was he saying when he was yelling? I’d have to agree with the cops at this point–sorry. Your story doesn’t work. I’m not saying you’re lying, but it sounds very much like it could be a family dispute gone awry. She’s pregnant, obviously by some abusive or worhtless man that she called her dad to come and get rid of. Men get quite upset when their daughters get knocked up by abusive men. Maybe your dad was trying to knock some sense into her and things got nutty, I don’t know.

And if you’re/her dad has a history of being physically abusive to her, then why would you call him to help out in this situation anyway?

If you want a man to leave your house (your sis’s boyfriend), and he won’t leave, don’t call someone who is going to make it worse (her dad). Call the police and have him removed. Maybe your/her dad is sick of her poor decisions and lost it. Maybe he’s an abusive jerkwad. Either way, the cops are right. You girls grow up and don’t call one abusive man in to take care of the other and expect not to get caught in the middle. Here’s an idea: just don’t call/talk to/deal with or make babies with abusive men. And like magic, these problems will disappear.