Claire Just one more word of advice, if I may.
Personally I wouldn't get involved in any tit-for-tat games, such as they-see-the-other-grandma-more-than-me-so-I'm-going -to be too-busy-to take-them over-there-in-my-car.
I doubt whether your DiL and her mother are deliberately trying to exclude you.
My guess is that they haven't a clue how you feel. Perhaps they are a bit intimidated by you, because you're a teacher and they haven't had a profession? Some people are.
Be the best, and most generous, woman that you can be(I don't mean financially generous) and then you will have nothing to reproach yourself for, and neither will anyone else.
Your time will come.
Remember yoghurt in the old days?
Captain Sir Tom Moore's charity under formal investigation.
Traders with too much work to care.
I wore my Beatles t-shirt to bed, and woke up, got out of bed, dragged a comb across my head...