I wouldn't say anything, at least not until things are much calmer, or at least worked through the feelings a lot more.
Norms vary widely. I don't recognise the groom's side/bride's side thing many are talking about as their norm or the childfree weddings others are describing as normal in their circles, but I get the appeal and why they're norms in some places.
Everyone makes their own choices. Once you've gone through your feelings, you can make yours clear-headed. It may be worth at that time discussing with your brother how he sees things going after he's married, how much time he expects to have with you and the rest of the family. That matters more than one day of a wedding.
Another thought- honestly would parents on here commenting that they wouldn’t be bothered, they have every right not to invite them, etc genuinely be happy in the future if one of their own children did this to their sibling’s future children?
My husband and I eloped and fully recommend it to our kids. None of our siblings nor our only nephew at the time even knew about it until at least weeks later. It was the best choice for us and for the time - we married young and knew people would give us shit, it was a bit harder to do that after (also, much easier to do on a tiny budget).
I have two siblings and none of us were invited to the weddings of the others, and certainly not the others' kids. I didn't feel disrespected by it - we just all lived very different lives quite far apart, and we all had very small weddings - possibly in part because we grew up with the norm of everyone possible being invited, expected to perform a role, and we all really hated it. I remember the last whole family wedding we were all at, and we spent much of it miserable - it wasn't the lovely joining of families, it was playing a part without our input.
And that’s the saddest part really, modern families are increasingly more distant and value friends over their own family
Modern families? This isn't new. My family tree is full of people who moved far away from their families for a new life, for love, for friends, for opportunities -- many never seeing or hearing from their kin again. I have more options than my ancestors, but it doesn't mean we're that different.
For me, what's sad is overly focusing on the wedding, as if not being part of the wedding means not wanting someone to be part of their lives. The wedding is one day (or a few days, depending on traditions and practices), it's the start, but not the end all and be all of what the couple's married life is going to be.