How do I nicely handle this? I say nicely because the words I want to say are probably not appropriate 
My fiance has 2 brothers, brother A and his wife are amazing, we get a long, we have never had issues with them. Brother B... him and his wife are a piece of work, they openly hate all of us for no reason (I'm really not being dramatic, his wife is one of those people that loves drama and has multiple enemies). They haven't talked to any of us exept his parents in 2 years, my fiance didn't want to invite them to the wedding but I urged him to ask so he doesn't ever have to regret not and we agreed they probably won't come anyways. Turns out they are! We're having a destination wedding and that's pretty much the only reason because my fiances family are footing the bill for everything. Free vacation.
Now my mother-in-law to be is like bending over backwards for my fiance's sister in law because they're coming. At first I blew it off but now it's just down right annoying. Now I found out from some of my fiance's relatives that his SIL was bashing us a couple weeks ago to a room full of people. We saw her at a family dinner and she wouldn't even look at me and didn't speak to my fiance either. Sooo my questions is, what is the proper way of handling a situation like this? I know I need to act like a grown up and ignore her, but a very childish part of me wants to say F* off, you're not coming to our wedding anymore! How do I let this women not wreck our day? I should also mention it is a very small wedding so there won't be a room full of people for me to avoid her with.