Need advice

[Zundfolge]

Simple fact is that as the most law abiding person in the room YOU have the most to lose when the police show up.

You touch this guy (unless it's a VERY clear cut case of defense with plenty of witnesses) and you will lose your CCW and probably your 2A rights altogether (and probably some freedom) and still not adequately protect your sister from him. Now is the time for you to do everything 100% by the book even though you really want to just drag this filth out into the woods and bury him (and I'd say you're justified for feeling that way ... just don't act that way).

So I'd let the police handle this ... if she refuses to call the police on him and she refuses to get a restraining order and/or take further legal action against him then the sad truth is that she has made her bed and she must lie in it.