When I was a teen, I went to Disney with a friends family. We stayed offsite, his two grandmas were FL residents and got us all tickets. I had to pay for my tickets, my food, my souvenirs, and a portion of the gas and hotel. I shared a room with his two sisters and one grandma, he, his parents and other grandma were in the connecting room) and was asked to pay for half the hotel room. It was made very clear at every sit down meal we did (not many) that I was paying for myself, and before we left I gave them an envelope of cash for 2 tanks of gas. At the time we lived about 7 hours away, so I paid half (or more) of the gas. Looking back almost 20 years I think that was a bit ridiculous, and it was awkward.
We took my niece when the girls were 8 and we paid for everything. If we didn't, there was no way that my niece could go, and we wanted her to go. It was a bad decision. lol Taking the niece, not paying for everything.
My daughter was invited to Jersey & NY with her best friend 2 summers ago. They drove, stayed at grandparents house and I sent money for food (she's a gymnast and can EAT), expenses/admissions/transit and souvenirs. Her friends dad refused to take any of the money and except for the little she spent on *things* - she came back with it all. I wasn't upset, and friends dad said my daughter is like family and he felt weird accepting it.
Now, it's true, the girls are like family to each other, and to the adults. BFF is an only child, my oldest is 6 years older than her two younger siblings (whoa re 11mo apart) so having a friend with her makes everything more fun. BFF has come with us to many things and we usually pay everything for her without thought, she goes places with BFFs family and they pay, never any worries. Both girls usually do have money with them just in case. And there are times I will tell all 4 girls - I'm buying one scoop of ice cream each. If you want a second scoop you're paying for it. My youngest will never use her own money for a second scoop of ice cream, my middle will buy it every time. So will BFF. My oldest varies.
We're all going to Disney together this June, then a
Disney Cruise. Because we are 5, and everything usually books in even numbers (or has space for even #s), adding BFF to our trip doesn't really cost us anything. So the plan was she'll drive down with us, get a hotel partway down, stay at Bonnet Creek with us until her parents come down, join them, play, cruise and drive home with us while her parents fly. Her parents will buy her Disney ticket and her dad will try to foist money on me for her meals that I won't accept. (It's the way we both are!
) She'll have a prepaid debit card that I'm supposed to, but won't, use for her meals, and she can buy stuff with. He resumes the cost of feeding her once they arrive. We've since decided to take the Auto Train. Her dad offered to pay her fare. We refused -with the special kids price and a AAA discount, the cost of paying for her ticket is less than it would have been to drive and pay for meals and a hotel - so I won't accept her dad's offered money.
I rambled - the point is - we know the family well, we've discussed and been VERY clear about who will pay for what and when and all parties are in agreement. I think it would be great for your 15year old to have someone closer to her age. But you have to think carefully about who you will and won't allow to go with you, how well you know the families, and what you can and cannot pay for. If everything is upfront, and all parties agree, then no worries.