The level of OTT drama and entitlement from the OP, her husband and her mother - didn’t her dad also say he wasn’t going to go now?! 🤣 bloody hell.
Elements of my family are dramatic and dysfunctional but just under the level of histrionic fortunately. I can’t imagine my parents getting that involved or threatening to not attend bc my kids weren’t invited to my brothers wedding. They wouldn’t be happy about it, but I think they’d accept it was his and his partner’s day.
Does your brother and SIL have any kind of a relationship with your kids, OP? Do they see them regularly or is it just once or twice a year, in which case they won’t have the same bond and connection as many ppl on here saying they can’t believe they’d contemplate excluding them.
If you have a remote relationship with your brother then view it as HER thing, and try to take a step back. It doesn’t sound like you have a particularly close relationship with her, if you do, I’m sure you’d have had the discussion with her or she’d have given you the reasons herself. If she feels an outsider to your family she would feel no loyalty to you and your kids or compulsion to spare feelings and invite them to keep the peace. If they don’t see much of you now it probably won’t affect them whether they see you or not going forward. Selfish, possibly, heartbreaking for you if you want more than that - absolutely - and I totally understand one-sided sibling relationships, believe me.
I really like the concept a PP said of weddings being about uniting and celebrating two families - that’s absolutely what they should be about. But if your brother has chosen this woman and she doesn’t want to be part of your family beyond being married to him there’s not a lot you can do about it. You can’t force ppl to care or want to spend time with you.
If this will really upset your girls then don’t go. If they aren’t that bothered and it’s more of an issue for you and your husband, I don’t think it’s likely you’ll go anyway. Your parents reactions aren’t helping things and you all may be seeing considerably less of your DB and his future family after the wedding unless this is resolved.
Side note - I’m surprised your husband is as bothered as he is by this. No one likes seeing their kids unhappy, but children have got to learn that life isn’t always fair and need to be taught how to become resilient to disappointment. His reaction made me think of the parent who rings in to complain their kid doesn’t have the starring role in a school show or hasn’t been invited to little so and so’s small birthday party. He’s probably feeding your emotions and resentment and in turn you are your parents.
It’s possible you may have been in denial over your brother’s attitude towards your overall family and this has brought into focus that he really isn’t that bothered and is and will be prioritising his soon to be wife. It may be better for everyone’s MH and well being to accept this, sad as it is, and take a step back.