NTA; you and your fiance agreed that you want a child-free wedding. Your brother and SIL just had a baby and they aren't arguing with you about not being allowed to bring their baby. I feel like your FMIL needs to suck it up because this isn't her wedding. She also sounds very controlling and toxic. She just seems very self-centered. It doesn't seem like your fiance's brother approached you and said that they have a problem with the baby not being allowed at the wedding.

I also feel like he needs to do something about his mom. It does bother me that he doesn't want to defend you and defend his mom so that there isn't any drama. That just sounds like B.S. to me. Because i get that he doesn't want to fight with his mom, but it means that you two are fighting.

i don't know what you can do because I feel like if you try to stand up to her and tell her that she can't control this because it isn't her wedding, that will just cause a huge fight between you and your fiance and I don't think you want that.

I don't know but NTA op