This, minus the threats. You have a right to let him know you don't appreciate the way he treats your mother and sister. But issuing threats or making attempts to run him off will only make things harder for your sister.
Assuming all you say about him is true, this is the kind of punk who will take your aggressiveness and turn it back on her. Not saying he's gonna hit her, but he'll find a way to punish her for whatever threat you issue.
Beyond that, unless she gives you permission, or unless he hits her, you have no real place meddling in her life in an underhanded manner. If you fail to run him off, you risk completely alienating your sister because she will most likely side with him (unless she's given that permission). If you succeed, you've put your sister in a position to have to raise a child on her own - which is fine, many can do it, but that's her choice to make. She's a grown adult and she needs to be responsible for her own life if she isn't in danger. If she isn't on board with all this and you run him off, she'll just chase him, and you will have gained nothing and lost a sister.
Not saying it's not rough. I dated someone for over four years and was engaged to a guy who was emotionally abusive and it was miserable for my family. There was no way out of that aside from my making the choice that I wanted another life, and if they had stepped in before I was ready to make that choice I would have shut my family out of my life for a very very long time. Not saying it's right or healthy, it just is the way it is.
Talk to him, talk to your sister....but don't do anything she doesn't say is ok, unless the situation escalates, and even then, don't do it behind her back.
Side note: Mitch, entirely possible mom really doesn't want him to say anything. Sometimes we're able to brush off insults much easier than the drama that comes from confrontation. I had a recent incident where I asked a guy NOT to get in another's face about something inappropriate he did. My guy friend didn't listen, got in the other guy's face, started throwing around threats like I was gonna take legal action (what the fuck?), and now those two and the other's wife are all not friends anymore. It wasn't worth it and I wish he had kept his mouth shut. The resulting fallout made me as uncomfortable as the triggering incident itself and had more lasting effects. Or give SB 10 million Karmaz to whoop his ass.
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