Ok, so here is my story, sort of like your own.
I had planned our Disney vacation just our family of 5. Then talking with our good friends, who were no living in NC (we were in MA), they decided to join us. With excitement, I was like perfect, we will have a blast. Our kids were 7, 5 and 3. There's were 3 and 2. So with that being said, I told her every place we were eating, what park we would be doing etc. She was like I don't think my kids will like that or I don't think they will do that etc. I kept our plans as they were. They ended up going with us to all the same parks but would leave about 3 hours into it. We just kept going and then we would all meet up again at supper. They would again, head out early. So for us it worked.
In fact, we are going again as families, this time us with a 9, 7, 6 and 2 yr old, and them with a 6, 5 and almost 2 yr old. We know we won't spend every minute together. Their kids aren't into meet n greets, mine are. WE love the characters. So we will do most stuff together, I am sure they will head out early and we will all meet up later.
So be selfish, make your plans with your parents, your inlaws and your family, if she chooses to join then fine, if not, she can catch up with you later. Tell her this is what we are doing bottom line. Good luck!