Yes, exactly. Another poster mentioned 'immediate family'. Nephews aren't immediate family in the same way brothers are. They're immediate family to you but not to your brother.
I guess that's the difference for me. I come from a very small family (I'm an only child and my parents have one sibling each, with my mother's brother only having two step-children). To me, my mother's brother is considered "immediate family". I've also been to more than one family wedding where no children were there, except for the children of the happy couple's siblings. For me, Aunts/Uncles/Nieces/Nephews DO qualify as that same level of immediate family that are must-invites.
I had absolutely no problem with it when I was invited invited to my second cousin's wedding and my son wasn't, even when it wasn't a completely child free wedding (the children of a first cousin of his with whom he is very close and who was in the wedding party were there). Neither he nor the bride had any nieces or nephews yet. On the other hand, if I had a sibling and they were to get married without inviting my child, I'd be bothered by that. While you can say that "they just wanted a child free event" and it's perfectly true, it's also true that part of the decision making process of deciding on a child-free wedding is that it's just particularly important to you to share your life-changing event with any of the children who might have been invited. It does send the message that none of the children in question are all that important to you, whether that's what you intended or not. If, as the OP said, there are only 4 kids in question, and only two of them are really old enough to understand (her two), then yeah, I can see the father feeling hurt on their behalf, and not wanting to attend himself.
What I will say though is that I can definitely see where something like this would be different between large and small families. If the bride and groom are each the youngest of six children and their combined siblings have 20+ kids between them, then the relationships
are very different from what they are if there are only two or four nieces and nephews. Of course, a lot also depends on the individual family dynamics, but for my, pretty small family, not inviting a niece or nephew to your wedding, no matter what the age would be considered a very bad thing.
My argument would be that an obligation can't exist one way. If you day it exists it should exist both ways. If you say it doesn't exist on one side then it is unreasonable to hold the other side to it.
But it does exist both ways - it would send a very strong message to not invite your brother to your wedding and may well damage the relationship forever. Similarly we are saying that if you don't attend your brother's wedding without a very good reason then it sends a strong message and may well damage the relationship forever.
I'm trying to make this really clear so forgive me if I am a little long winded; Brother/brother is a different and usually much much closer relationship to uncle/nephew. I love my nieces dearly but I didn't grow up with them, they didn't share the ups and downs of my childhood and I don't turn to them in times of trouble. I don't have an adult relationship with them because they aren't adults. They really *don't* care about the state of my life, finances and marriage and they really shouldn't, because they're children. However if I was on the brink of divorce or struggling with debt or something I know my brothers would care and offer help if it were possible/relevant. If they didn't I'd be deeply hurt. Similarly attending my wedding (hypothetically, I'm already married) would only be a chance to dress up and go to a nice party, to my nieces. Attending my brother's wedding was a beautiful and moving occasion where I was overjoyed to see him so happy and deeply pleased to welcome my new SIL, who I love even when she drives me mad, into the family.
I hope this helps you see why to me, and perhaps other posters, not inviting a nephew is not even remotely comparable to staying home from a loved brother's wedding for no good reason other than to sulk over the guest list. That is why I, along with other posters, suspect there MUST be some kind of backstory.
ETA - typed all this up then read the update. Sounds like you have a plan in place, so good luck with it BrisVegasGirl.
I guess this is the difference for me. My maternal uncle is 10 years younger than my mother and only 15 years older than me. In many ways our relationship
bears more similarities to that of siblings. He also, for various reasons reasons that aren't really relevant to the discussion, didn't get married until he was in his late 40s, and when he did so, the ceremony was in my house, with me as the best man... so that probably explains at least a part of why I see that relationship
as a more important one than a lot of other people seem to.