Does your MIL know/understand your scheduling constraints?
We have told her, but I honestly don't think she understands, not really. I think that she thinks we are just giving her excuses.
I think that sometimes the custody arrangements are something you can't wrap your head around unless you're living it. They're usually not all that straightforward to begin with, and then when you have to work out scheduling for the summer & holidays, it can be even more difficult for an outsider to grasp.
However! It's not really important for her to understand the nuts and bolts of it. It's only important for her to understand that
a) you don't have your kids all the time
b) there are times you don't get a choice in the whether you have them
c) even when there is some flexibility, it's still not totally in your control
At this point, I think it's beyond trying to explain it or trying to make her understand. In your situation I think I would just say, Look, I know it doesn't make sense to you but this is the way it is. You can keep fighting windmills or you can accept it and work with it.
To be honest, I wouldn't really care if she thinks we're making excuses or being deliberately difficult or whatever. Let her think what she wants. She is causing her own frustration at this point. Whether she likes it or not, whether it's your fault or exes' faults or the fault of the alignment of the stars really doesn't matter.
And that's why you don't JADE.
These are the weekends we are free this summer. If you can come on one of them, great! If not, oh well.
I think it's one of those lather-rinse-repeat situations. You just keep saying, this is when we are available and that's not going to change.
She'll either catch on or continue to be unhappy till the kids grow up.