I don't get it either IF they can afford to go. My sister and BIL for instance...have 3 kids (almost 8, almost 6 and almost 3 year old). Every year my sister comes up with a new excuse as to why they're not going with us. They have no problem buying a new drum set, laptops, kindles, etc etc. They never take their kids anywhere or do anything with them, I feel bad for them. The new excuse is they want to wait until the youngest will be able to remember it. Ok, I get that, but what about the older kids? Remember it? I only remember bits and pieces as a teen, so I'm not sure when they plan on going, lol.
They don't like Disney - frankly, it doesn't sound like they like to travel.
Every year I get invited to a Superbowl party, and every year I have some excuse to not go (its often Girl Scout Cookie Go Weekend - that's been a good excuse for a few years). I don't like it. I don't even like the ads.
I can't imagine being pressured to spend my vacation time and dollars doing something I have no interest in.
Your sister and BIL don't like to travel. Some people don't. When their kids are adults - or even teenagers and get opportunities through school - maybe the travel bug will bite them. Maybe it won't.
My daughter's best friends have a father who doesn't like to travel. They don't go many places. On the other hand, they have spectacular DREAMS about being able to travel. Which, in some ways, is cooler than my "I've been on three continents" daughter.