I think the answer is that unless you MIL is paying for this vacation, which she's not, you don't have to include her.
I would never vacation with my MIL. DH has mentioned it a few times, and I've politely told him that one of us (me or his mom)wouldn't be making the trip back home.
I work too hard to save for vacations to include someone who would ruin my trip.
I hope things work out in your favor.