You guys either have some REALLY inconsiderate families or else you need to learn to say no!
I am a new owner, but it wont matter. We will go on vacations and sometimes offer points to people, but it is ALWAYS going to be when I want to, not just because I am asked. I love my siblings and family very much and we vacation some together now, but ALWAYS at each individual family's expense.
As far as covering extra expenses, I would pay ONLY if I were taking a niece or nephew with us (not the whole family) and I would hope that their parents would at least offer to help out (in which case I would make sure they just gave the child plenty of play money - not true needs money).
I consider us VERY close. Maybe it has to do with respecting boundaries.