I've been there, done that, and trust me when I say you definitely need your own family time!
We went with DH's ENTIRE family in 2004 - 19 of us. My SIL planned the whole thing and the rest of us were clueless and just went along. We stayed in 3 cabins at Fort Wilderness, went to every park together, had every meal together, etc. etc. It was awful. WAY too much family-togetherness. And we wasted a lot of time as we were always waiting for this to one to use the bathroom, or that one to grab a drink, or waiting while one kid was having a meltdown, etc.
Last year, we went again with my BIL and his family and agreed ahead of time that we would NOT have a repeat of our prior Disney vacation. We planned a couple meals together and stayed in the same resort, but that was it. If we wanted to meet up at a park, we did. If one family wanted to eat here or there, they did. Some nights we met in our resort (BWV) for some evening drinks. It made for a 100% better experience all around for everyone
Definitely have this conversation before you go. I would also encourage you to tell her to do her own research on the parks, restaurants, rides, etc so she isn't expecting you to plan every day for her. While it's great going with a Disney expert, that can be very stressful on you if your plans don't meet their expectations. And not fair to you at all. You need to enjoy your vacation too!
Good luck!