Are you confident that your sister's safe with this guy? That didn't seem to come up,and this kind of stress can have a bad effect on a pregnant woman.

Anyway,this guy sounds like a real lowlife,and I've found the best way to deal with that kind of person is to be direct. Not rude or violent,just direct. Try to stay as calm as possible and don't let your anger cloud your words. Frankly,if the guy's as immature as you say,he may just break down and start crying about it. My sister's ex-boyfriend was just like that and did the exact same thing when I set him straight for telling my grandmother to shut up.

But I would agree that you do try talking to your sister about it first,though,and attempt to let her try to settle the situation (the reason I inquired as to whether you thought she was safe around the guy).