Sounds like some legal action needs to be taken. Also, some counseling is needed.

And a good whomp across your sister's head would be good too. What is she doing hitting her mother?! That's ridiculous. Oh boohoo, she didn't get what she wanted, go cry me a river because I don't care. That gives her no right to hit your mother.

You can't make your sister stay with a guy she doesn't want to be with (her old boyfriend). Instead, you must move on and let her make her own choices. You, as a sister, must do that. And your parents can't make her be with anyone, but they can stop her from being with certain people until she's legally an adult and can support herself living on her own. Sounds your like parents are just allowing her to get her way. :/ Talk to your parents about what's going on, and how it's making you feel. Sometimes, a little tattle-taling is good.