I am the planner for our family trips. We did take one trip that was a BIL, SIL trip and they love Disney as much as we do. It is hard to get everyone's bladder's on the same schedule, for one thing. On the first day, we spent the majority of it, in front of bathrooms, waiting for 2 or 3 to come out. On day 2 the same thing started, my Husband said,"
STOP" Everyone go to the bathroom, there was some whining, we were 11 people, he said, "
GO" We all went, It ended up being one of our best trips, we all started the day with what we each
really wanted to do or ride or where we wanted to eat (this was before ADR's were a must), we each got to say one thing, then we did that one thing for each person, We cried when the trip was over. We had traveled from different states to travel, to the Magic. I know what the OP is saying, I have had some difficult inlaws. When the Lion King was released, at the Theatre, The Bird, can't remember his name, talked about families having difficult family members, he said," I have two." I laughed so hard! I said, even in a Disney movie. My SIL, said, " I knew you were going to like that part."
I do think that the Planner appreciates the trip more than the ones just attending, I would give her a book, for sure. That way, she can not blame you for not enjoying herself. If they stay offsite, they cannot get the
DDP. The Op did not say if her family was, that could crimp the style of the trip.