I would do the math two ways. Split it 6 ways and 4 ways.
You and #2 each would pay 2 shares, and #4 and #5 each pay one share.
Then I'd tell #2 that her cost is __, the discussion is closed, and if she doesn't want to pay that she can stay home.
I'd plan on splitting it four ways, but I suspect she'll cave eventually.
Because Dad and #3 lives close, can't one of them pick #2 up at the airport? Why does #2 need a car? Assuming dad and #3 each have their own car, #3's contribution can be to let #2 use her car for the week.
Id vote for choosing a vacation that everyone can afford to begin with, including #4 and dad. We are comfortable financially and just dropped $1k on a vacation with my ILs, and I wasn't too pleased about it. Just because you can technically afford something doesn't mean you should be expected to willy nilly.
Maybe talk to the ones concerned with the costs or who are not able to afford it about planning something that everyone can fit into their budgets. And if the "Obama rich" 1 (whatever the hell that means) wants a more $ spot then they can defray the difference.