Not Sure How to Proceed

I would take her off the guest list! My personal preference is to only invite people who my FH and I are close with and this includes plus one's unless someone is married than I would of course include their spouse or serious gf/bf. However, I definitely would NOT include someone who has disrespected me, my close friend or future sister in law. I'm sure my FH would agree. This is one of the most important days of your life so don't hesitate to uninvite toxic people who have threatened to fight you.

I've dealt with people like this, there is no point in trying to "talk it out". They have their own agenda and if she's this adamant about behaving like this towards you the last thing she is probably willing to do is talk.

You need to discuss this with your FH first and foremost because whatever decision you make should come from a united front.

If it were me, i'd definitely tell the groomsman that she can't come because it's just going to cause drama all night and you don't want that. If he decides to drop out then that's his choice. But, i'm one of those people who has zero interest in dealing with people like that so my choice would be harsh.

She doesn't deserve to be there PERIOD. Your fiancee should tell his friend that and why, and if the groomsman backs out it sucks but from the sound of it he's no prize. I say cut her out of the guest list. I would without a second thought. I'm to old for drama and games. She is not your friend, has proven that more than once, and I get the feeling she'll do something to disrupt your wedding if she is there.

WHAT?! As grown adults, this is actually the most absurd thing I have heard. It is YOUR special day and you have every right to no accept this girl's presence there. Talk to the groomsmen about ti and if he doesn't understand, then would you want him standing up there anyways? BYE HONEY.