Is there any way to make it pretty clear if he can't come to family functions and mistreats her as he does AND that the tickets were supposed to be for her 2 kids and her, that we'd rather he not be there?
Yes, the direct way. I guess you have been avoiding having this sort of convo up until now.
Myself, I couldn't go to this concert with that creep there knowing he'd hogged the ticket you got for your niece. No way it wouldn't get fairly ugly, fairly fast, like before we ever got to our seats.
But then, no way I wouldn't have already had this kind of convo with my wife's sister, had I been you, so . . .
It's kind of hard for me to take myself out of this situation and give you the clearest, kindest advice on what you should do now, given you haven't done what I would have done long before.
Not trying to tap dance on your head, OP, I truly feel for you, I do.
If you can stand going w/o saying anything beforehand (I couldn't), then try and make the best of it.
If not, the convo starts with your wife's sister, your wife, and you first, absent Billy Bob. Do NOT involve your niece in this, the person who gave you that advice is weetarted. You do NOT want to have a heated discussion about your wife's sister's man choices in front of your niece! (I think you may already know that, though.)
Only IF you three adults can present a united front can Billy Bob be directly addressed, and this should be first done alone by your wife's sister.
Only if she then can't get Billy Bob to see the light and relent should
she then tell him he can't come because you and your wife, who got the tickets for her and her two children, do not want him there under the circumstance that he attends with the ticket meant for your niece.
If he still won't see the light, damn, you're fucked. Me, I would personally go so far as to plan an alternate event for the five of you that evening and make Billy Bob go alone, but then that's just me and may well not be you.
I just couldn't go to that concert if he came.
Thinking about it some more, if Billy Bob goes, this is what I would do, if your wife and your wife's sister and her children would all agree. All FOUR of them show up (the niece comes under whatever story) and meet the two of you. You then tell Billy Bob what's what, that he can either do the right thing and get his own ticket or the other five of you are going somewhere else. Make sure you have every ticket (save
maybe his) in YOUR hand before you say this, so he can't swipe the other tickets from your wife's sister. Stay calm but focused. Let any misbehavior start with him.