Hmmmmm...either he has serious wedding jitters, or you have something to worry about.
Prolly both, or so my "waters" tell me.
Look, if he's gonna be an abusive ****, he's gonna be. I am getting bad vibes....but heck, I have wrong a lot before in my life.
As you likely know, the first tactic is usually cutting off partner from family and friends.
For what it's worth, (and I have lived through this with a couple of friends, plus a zillion times with clients but that doesn't necessarily mean I know anything helpful to you) I think your aim is either not to get cut off if possible, or, at the very least, to let your sister know that, no matter what happens, you guys are there for her, and she need never feel bad about coming to her family. You'll be there.
This means you walk a fine line re raising concerns about him with her.
If she's in love, she's half nuts right now....chemically speaking. I'd be careful about making any overt criticism of him...even if she does. If she goes through the yo-yo thing with him that a lot of folk do in bad relationships, she'll resent you for anything bad you say about him on the outswing when she's back on the inswing.
I WOULD, however, be very empathic with her tears and worries, and maybe gently ask if this behaviour is of concern to her. Kind of let her know you are concerned, too, without actually saying anything really overt....unless you get the vibe that she is really worried, in which case I'd just quietly make it clear that cancelling the wedding, should she wish to do that, is something you'd all help her deal with...but be careful, is my advice. If you are like me, you are aiming to still be there if the **** hits the fan.
Your being in a semi-parental role perhaps makes it a bit mine-fieldier?
I'd be trying very hard to maintain contact with her even if he tries to make this very difficult. This might mean swallowing his crap a bit more than you normally would.....remember, if he is abusive, he'll be out to get rid of you and isolate her.
I am likely just telling you how to suck eggs, in which case I am very sorry.
And I also hope like hell I am way wrong, and they'll be damn happy.
Believe me, I do wrong often enough!