Maid of Honor brings up ex husband a lot

Hi!


So I'm getting married September of this year (2020) and my maid of honor is my cousin. We use to be a lot closer but we both became divorce single moms around the same time 4 years ago and have since drifted apart due to busy lives and distance between us (she has always lived 2 hours away but kids make it difficult to make trips out for the weekend). We've managed to stay relatively close but it's been a rocky couple of years. However, in the spectrum of a lifetime...the last couple of years seem small.
So it was just assumed she would be my maid of honor. If I'd chosen anyone else I would have lost her friendship...it is that kind of relationship. I did ask her before hand if she was up for it because she's been really busy lately and I didn't want to overwhelm her but she said she wants to so OK 🤷‍♀️.
We had our bridesmaid lunch this past Sunday where we all got together. There are 5 bridesmaid and then myself. My cousin instantly picked a fight with another bridesmaid whom she has never met...granted...this particular girl (we'll call her Krystie) is a tad bit assertive and so is my cousin...so...I had worried that they would clash. So that was awkward...and my other bridesmaids noticed it too...it was pretty obvious that my cousin was being SUPER catty to Krystie. Krystie does do the annoying thing that my cousin was accusing her of...but that's still absolutely no reason to be rude and catty like she was...
But that's not what's got me super stressed out. Like I mentioned earlier, I am divorced and I have a son who is now 5. My cousin knew my ex husband, didn't like him but they got along when they were around each other for my sake. And my cousin knows my fiance...we've been together 2 years now and we've all gone out several times over the last 2 years. She knows him well...his name is Josh, but she kept calling him John at the lunch. I mean at least 4/5 times. She's never done that and that was confusing...
Sorry I'm taking forever to get to the point. Then...we were talking about details and I said when everyone exits the ceremony after we kiss, I want everyone to dance while leaving. I think it would be fun. She said instantly "I'm not dancing." Some of my other bridesmaids said "come on, you can do it for one day! It's for your girl!"
This is what kills me. My cousin (maid of HONOR) said "I've already been through this with her once! This is the second time!"
OK...so...yes. She has. But...that was a thrown together last minute "wedding"...I didn't have a bridal party...and I didn't even have a venue or reception. We paid a random pastor $100 to marry us quick (I was not pregnant) and barely had 15 people there. That's why I want a big wedding this go round...I airways regretted not doing it and never thought I'd have another chance. She has a tendency to say inappropriate things and now I'm worried because I don't want that brought back up. It isn't fair...he cheated and left me and my son and that's not something I want to think about while I'm planning a wedding. And I'm freaking out about the maid of honor speech.
I don't know how to confront her about this. She is so hyper sensitive and she will get pissed and just leave...or decide she doesn't want to be in the wedding or attend. I have already called "Krystie" and apologized for what happened at the lunch...and I need to talk to my cousin about that too...but what really has me stressed are the comments like that. If you were in my shoes how would you handle it?