It also doesn't work.
Dm used to do a lot of the "you're such lucky children whose parents care so much that they get you a lovely healthy packed lunch and those poor children who don't care get a unhealthy meal bought at a café. Aren't you so glad you're out here freezing in the rain eating sandwiches squashed from the bottom of the cold box watching the children inside having hot chocolate and ice cream in the warm and dry ."
All it made me feel was that dm was deliberately making a choice I didn't like and I was being mean to wish that I could have something different. I'd have far better heard that we couldn't afford it, and then it wouldn't have felt like it was done deliberately because they thought that was best.
I do think that the replies of how mean granny is being, how nasty the cousins will be and how unfair it is for the brother to talk about what he's doing frustrating.
It's more than possible that the ds is asking to see the photos. When dd2 was about 7 or 8 I picked her and a few friends up from a club. I can't remember why but I told them something from my childhood and after that every week they leapt in the car calling "please tell another story about when you were little". I told the most boring stories at times, and they were absolutely riveted on every one of them. Heck, the story of walking back from Brownies with my dad pushing his bike was one of their favourites - nothing happened!
He may may want to see the photos and just be interested why they don't do these things rather than jealous.
It sounds like the uncle is sharing with granny, not trying to rub it in their faces, so not doing anything wrong.
We've never been abroad. Ds is about to go for the first time with his uni friends. Last time I went abroad was in 1990 for a school trip. When the children were little, the holiday their friends wanted to do and envied was our Butlins trip. They'd gone abroad and staying in posh hotels. They wanted to go to Butlins.
Children don't care about the expense. They care about the fun they can have. It's certainly possible that you might find that the cousins are saying after a trip to Granny's "why don't we ever camp? It's not fair we stay in the boring hotels. I want to stay in a caravan..."