Sorry this has got you wound up, the crazies in our life tend to do that to us.
I don't get along with my MIL, so we generally do not speak other than the most basic pleasantries, but one thing than has helped for us is that she calls every Sunday at 6:30 PM to talk to the kids.
This is a time we decided together would be the best for all of us, the kids know that is when Grandma is going to call, so they set that time aside, the older they get the shorter the call is, but they still make the time.
If Grandma misses her call time, then that's it for the week, we don't jumble the schedule to make time for her, she waits until the following Sunday.
It has worked extremely well for us, there is no conflict because the rules have been set for everyone in advance.
I know it is not your job to make sure the kids talk to grandma, but if you all could come up with a system that works for both of you it might make your life easier.
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He who would be serene and pure needs but one thing, detachment...Meister Eckhart
Serenity is not freedom from the storm, but peace amid the storm...Anonymous