Does your sister have a child living in this violent household? What you sister should probably do is take her child and go to a shelter. Single mom with a young child no car and a violent household she’d get help from the shelter. If she doesn’t have a car, she should call shelters and they’ll send a car for her, like a taxi.

The only thing I don’t think she should be doing is hitting anyone with a vase. If she gets law enforcement involved it might look as they both are in a violent brawl rather than her being attacked. She needs to make police reports and call police every time she’s being attacked. I also don’t think you need to jump in The middle of the fight but call police instead

Could she leave her child with you or other relatives? She should not have children living in a violent unsafe home