@DestinationForOne
This isn't about the wedding, this is about MIL trying to reassert a hierarchy where she is the important one, her daughter is the golden child, her granddaughter is the “elevated” one and the sons are expected to fall in line
while wives/partners and grandchildren are treated as optional extras. Your DH stepping out of line, even politely and it triggers a loss of control for her. That’s why she’s reacting so disproportionately. They’re not upset about the wedding, they’re upset that he’s no longer complying with their hierarchy. That’s why the reaction is so extreme. He’s allowed to decline. They’re allowed to be disappointed. But they’re not allowed to punish him for prioritising his own family.
In MIL's mind compliance = harmony, boundaries = conflict, wives = competition
children = distractions, the golden child = untouchable. Your DH is saying “no” which seems to be the first time he’s disrupted the pattern. That’s why it’s WW3. Buts its a really healthy thing for him to do (all though it might not feel like it) and very supportive of him.
“We want him there as a son, not a husband or father.” This is not normal. It’s an attempt to de‑prioritise his nuclear family and pull him back into the role she prefers. It’s also a subtle way of saying: “Your wife and children dilute your loyalty to us.” and also that they come before you and his kids....except his niece's marriage never should come before his his kids and their mother.
Excluding the spouse and children from a family wedding - This is not “just how they do things.” It’s a deliberate boundary: “We decide who counts as family.”
“Other wives accept this.” What she is saying is : “Everyone else complies. Why aren’t you?” This is classic enmeshment pressure, tell her “Other families make their own choices. We’re making the right one for ours.” This stops comparison dead. “I need my children there. ” She doesn’t mean all her children. She means: “I need my children in the roles I assign them.”
The TTC issue - the posters who say TTC “isn’t relevant” are missing the point.
It is relevant because it could be a high‑risk pregnancy, possible newborn, five‑day trip abroad, emotional stress, and physical limitations. And he may need the 5 days at home. Your DH is allowed to consider his wife’s health and his children’s needs. That’s what a functioning adult does.
The ‘next of kin’ comment, well that's not just odd, it’s revealing and a bit control freaky in my mind. It shows she sees herself as still the primary woman in his life, still entitled to make decisions still central in his identity and that's not healthy.
Your DH isn’t being unreasonable at all. His parents are trying to pull him back into a role where they get to define who “counts” as family, and they’re excluding you and your DC because they want him focused on them. That’s not normal, and it’s not healthy.
Your husband doesn't owe them his personal life and your private details but he does need to set boundaries going forward. He needs to say something like
“I know the wedding is important to you, and I’m glad you’re excited about it. But I’m not able to commit to travelling abroad next June. We have things going on here as a family that mean I need to stay close to home. I’m sorry if that’s disappointing, but it’s the right decision for us.”
If they keep pushing he can add something like “My wife and children are my family. Any decision I make has to take them into account.” and/or "“I’m a son, but I’m also a husband and a father. All of those roles matter, and I won’t ignore one to satisfy another.”
Good luck OP!