I agree with the posters. Ask the parents what they want. Do they want a reasonable trip where all the siblings can attend or would they prefer going to an exotic destination with only 2 of the siblings.
It's totally and completely reasonable for you to opt out of the trip, be it for financial reasons or you just don't want to leave your kids for a week. Or maybe you want to save the money so you can take your own kids to an exotic destination.
Frankly, the folks who should feel bad about disappointing your in-laws are the selfish, self-centered siblings pushing for a trip that not everyone can attend.