His ex…

This could be anything from your fiance's ex simply being jealous, to her possibly having a personality disorder, and everything in between. Personally, while her behaviour has been unsavoury, I wouldn't overthink it.

I understand how this feels.
My ex husband and I have been divorced almost 5 years, he got remarried in 2019 and I did back in October of last year. I of course had heard about who he was with long before they thought, of course because of my son talking about when he is up with his father. When I initially met her it was during an exchange, which in my opinion is always the worst way to meet the "new person". I didn't say anything to her, as she would remain sitting in the car, so I asked my ex if that is the "new person" who our son had been talking about. At first my ex told me it wasn't any of my business, but eventually did confirm. It was a couple days after my ex proposed to her when our next exchange was and I decided to try and be cordial saying " hi and congratulations", but she was not having any of it, as she completely ignored me and kept trying to put the car in drive (from the passenger seat) telling my ex to drive and it's time to leave. At the next one, all of sudden she became the absolute nicest and sweetest person who could have ever imagined. Very upset in regards to her previous behavior, repetitively apologizing and hugging me, saying that she shouldn't have acted that way in front of my son as that was not the example she wanted my son to see and for me to know her as.
Of course it was absolutely fake, nothing about her "apology" was sincere, and she continues her horrible, selfish, carelessness behavior and talk in my presence and in front/to of my son.

It is so terribly sad that whether these people are the ex of the previous relationship or the "new person" this behavior happens, and unfortunately there really isn't anything that can be done by one person in order to fix or help this. Apparently even after 8 years of being divorced and even being remarried herself, she has never moved on mentally, and has become overly jealous of the fact that you and her ex are moving on in life with getting married. Sadly I do not see anything else you could do or say that could help "change" her reactions/feelings. It is something that she has to be willing to work on in order to better herself.
I am sorry to hear that she is acting this way towards you as it is very wrong and immature on her part to do so.

I mean, she is remarried which would effect it just as much as him remarrying lol.

Kristine - Don't worry about your relationship with his ex as long as you are polite it doesn't matter. From the sounds of it the kids are old enough to make up their own mind about you and hopefully you are good to them and it's nice they are bridesmaids!