I am honestly as pleasant as I can be, supportive of her children. It feels like she badly needs someone to reject. She knows I love her and will put up with a lot. Will other mothers whose adult daughters have rejected them please respond? Thanks.

I will be honest with you, no disrespect intended.
But I think there is MUCH more to this story.

So "pleasant as I can be" sounds like you are not always pleasant in her home, can you tell us about when things are bad and what has happened in past?

Next you refer to them as "her children" not your grandkids, is there a reason?

Do you see your grandchildren other places ?

How often do you invite her to your house?