I know she has a right to be upset? I don't know what to do?

Time for your daughter to realize the world doesn't revolve around her. Sorry, but she sounds like a spoiled brat throwing a temper tantrum. Why do you allow your daughter to have any input to how many kids you have? Sometimes a 15 y/o needs a mom more than a friend. Tell her you are sorry, but this year the trip will have to be rescheduled.