I hear you Gary, and I don't envy your position. As a dad, I understand not enjoying having no control over situations that involve my kids. Also, the expectation of such a trip with no dialogue would upset me as well.
For a personal narrative, my in-laws go to Hawaii every summer, and book a house for us all to stay in. MiL was born there, and she took my wife and her sister there every summer when they were kids, so it is a longstanding thing with their family. There is an expectation we go with them, which is easy for my wife who doesn't work over the summer. But I have limited time off, and even so it is always an issue about me "selfishly" not wanting to spend all my vacation days on a trip with her family. I've been told I am not grateful for the gift they are giving us. I try to respond that while I appreciate the generosity, it would be nice if I had a little more say in how I spend my vacation. I remember last time there, over the course of 6 days, on one day, MiL said "So Chris, anything you would like to do today?" It stood out to me, as it was the only time anyone asked me that the whole week.
I have no idea what I'd do in your situation. I mean, of course I know, I'd go, just as you are doing. Try to go with good spirits though. I always have to psych myself up for family trip with the inlaws. It's not how a vacation should feel, but if I don't, I'll be a drag on the rest of the family and ruin their enjoyment.