I've been following along in this thread, and I just have one thing to add re: MIL feeling bad that she doesn't get to go on the Disney trip while the other grandparents do.
In my relationship with my brother and SIL, my family-of-origin is in an equivalent situation as Slarti's parents/siblings. In other words, my brother lives less than 5 minutes from SIL's entire immediate family (her parents and sister), but my family-of-origin lives scattered across the country. We represent 4 different states in 4 distinct regions on 3 coasts. As a result, I and my sister and my parents don't see my brother and his family anywhere near as often as my SIL's family sees them.
So from my perspective, I deeply value, appreciate, and kind of hoard the chances for DindraneFamily trips, distinct and separate from SILFamily trips. Because proximity breeds familiarity, any time the DindraneFamily spends with the SILFamily kind of ends up feeling like a SILFamily vacation...whether it's at my brother's house, my parents' house, or somewhere else entirely. It can be very nice to do, because I do like my SIL's family, but it can also be sort of frustrating to have the DindraneFamily subsumed into a larger entity every single time we see my brother.
So were I Slarti's parents, I would very much value a chance to see my daughter, son-in-law, and grandchildren in a situation that didn't involve their other grandparents every once in awhile. Even if I liked the other grandparents and enjoyed seeing them, it changes the dynamic for the far-away family to visit while the close-by family is also present.
I don't know that my SIL or her parents would see the situation in my family the same way I do, because they are on the other side of things (so to speak). So I can understand that Slarti's MIL might not be able to muster that particular brand of empathy in this situation. But whether or not she can understand that Slarti might sometimes want to spend time with her parents without her in-laws there, it's not reasonable to put any hurt she might feel in that situation on Slarti. So it might hurt her feelings not to be invited on this vacation, but no matter how understandable that is, it doesn't make it reasonable.