I would be disappointed, too, jojojo. Imo, this is between ods (older dear son) and yds. It's really not your place to say anything. Ods and dil have made their decision, yds and his bride have accepted it (what else could they do?), and that, I'm afraid, is that.
" ... son and wife keep themselves quite isolated and don't bother with family birthdays, get togethers..."
So it was doubtful from the beginning, imo, that they would agree to make this wedding, especially given the distance.
"I so wanted to show my GK's to the wider family who never see them..."
I'm sure I would feel the same way. And I know it's hard when one wishes for something that just isn't going to be. But obviously, ods and dil aren't concerned about whether the extended family has met the gc yet or gotten to spend much time with them. Maybe when the kids older the parents will be more interested. Until then, I'm sorry, but I think you'll have to table that wish. It may come true eventually, but not very soon.