Originally Posted by Carousel96
Well, if we invited the friend to dinner, we'd pay.
If we invited her to the local amusement park for the day, we'd pay.
I'm not sure that a vacation is any different in concept -- it's just the price point.
I wouldn't invite a friend to go along unless I could pay her way.
Personally, this would be my plan as well...if I can't pay, I don't invite another to join us. The "guest" should provide her own pocket money for souveniers, though. However, I'm sure opinions vary on the topic.
You don't indicate how old the girls are or what your opinion is of the other family's ability to pay. That could definitely impact. If you know the other family doesn't have money for this kind of trip - definitely don't invite unless you can cover it. Even if you "think" the other family has money, you never know what their actual financial situation is and a vacation may be out of the question in the time period you are planning, even if you are of the impression they have "good" jobs and can afford it. You never know what hidden expenses some families have.
Now, if the girls have cooked up this idea on their own...you might approach the other family such that "we've been considering a vacation and I understand your daughter would love to go...we'd be glad to have her join us but she would need to pay X plus her spending money."