Why do some grandparents seem to forget what kids' schedules are like? Even without the blended family situation, kids have sports and lessons and friends that they want to see. Not to mention projects that they need to complete for school.
Addressing just the issue of grandparents not remembering what kids' schedules are like--things have changed.
I'm approaching grandparent age myself. Back when I was a kid, I had one or possibly two after-school activities--Girl Scouts and piano lessons. That was the norm--there were more stay-at-home parents, so most kids went home after school and stayed there, or went next door and played with the neighbor kids. We didn't have to arrange play dates because there was always someone around to play with, usually lots of kids. And, quite frankly, there were almost no organized sports for girls.
Summers, again, more stay-at-home parents, so most kids stayed home and didn't have the variety of camps that many kids now attend routinely.
The children of my generation had more activities than we did, and the current generation has even more.
So many grandparents of today are thinking back to their own childhoods, and that of their children, and not realizing what's the new norm for today.
Add in custody issues and who has which child when, which was around but very rare back then, and the grandparent of today has no real clue.
None of this excuses the MIL in the OP. What's the saying around here? "When you make plans without us, you've made plans without
us." If she really wants to see her grandkids, she first must make them the priority.