Major Vent - family possibly not showing up - long

Fh has a horrible cousin that words can not fully describe. She has done horrible things, she’s said that he is dead to her, and pretends both of us are not there are family parties, and she is jealous thinking the oldest cousin should get married first so she moved 4 hours away to move in with a guy she barely knew.

Fh and I already stated(to fil’s) we don’t want her live in boyfriend, and the more we thought about it we don’t want her there either. Fmil is strongly opposed of this because she doesn’t want to upset her sister and the family (the cousins mother is also very jealous and manages to insult us at every family party). We think that if we invite both, or just cousin to the wedding that they will try and announce something like an engagement to take away from the wedding(tacky but that describes what they would do)..
Fmil says that if we don’t invite them that, that side will not show up, and that his other side might not show up either. So now fh is going to have to kinda find out how true that statement is, or if it’s a bluff. So the more Im thinking about it, if they know our side of the story and still decide to boycott, then I know im not going to have much desire to ever see them at family parties, Because shouldn’t they just want to be there at our day, and shouldnt our wishes matter?

Neither of us want her there, and we know she would do something at the wedding, and fh said he doesn’t want anyone messing up all my hours of planning & money spent(sweet of him to say). Even if she doesnt say something we dont even want to look over and have to see her at it... So shouldn’t the family respect our wishes on the day we will remember for the rest of our lives.?