If your daughter doesn't want her to go, she needs to speak up and say something. We have been on several vacations with my in-laws and they have been everything from manageable (with loads of boundaries and pre-talks about expectations) to completely awful. The last time my mother in law tried to invite herself on one of our Disney trips, I told her no. Was she upset? Yes, and she played the "but I want to see the magic in my grandchildren's eyes" card. I still said no. Your daughter is an adult and she needs to learn to have boundaries. I try to not be a jerk to my MIL, but I also am not afraid to tell her no. Especially something like a vacation that is going to cost a lot of money and could be potentially ruined by differences in opinion and expectations.