In life 'Tides goes up and tides go down.' Yes, you don't have to get sucked into other peoples dramas, but be patient and loving. Don't give up on your sister. Or for that matter don't give up on anyone.

If she’s with an asshole she’s going to need you more than ever. Just keep reaching out and let her know you’re there and always will be. If this guy is abusive and controlling, let him know that you’re not going anywhere and that he doesn’t get to control you. Whatever you do, don’t give up on her.

As someone in a similar position I can tell you that you can remain in her life without being around her hubbie besides family events (and sorry that's just the reality of inlaws). He'll have to problem showing her who he is over the time the same he's done to everyone he's not marrying lol when that day comes she'll need her sister. Sister's are such an incredible gift to eachother, I'd be so lost without mine.
Also sorry to hear that!