Sometimes, (IME) grandparents resent children going to a childminder because they are worried that the child will have a better relationship with the c/m than with them.
They feel threatened, that the child won't love them anymore and often the child does have a better relationship with the c/m. I encourage grandparents to visit and see what is on offer, and assure them that they will always be special to their grandchild, but sometime a c/m is the answer because of what is on offer etc.
I personally do not think grandparents should mind their grandchildren full time, they have done their bot with their own children. Now is their time and time to enjoy themselves and do all the things they couldn't do when they had small children.
I have had several children come to me because the grandparent thought they could dod it all, but after a few months realise what a comittment it is, all day, every day, not seeing their own friends for lunch, golf, shopping, spur of the moment trips and holidays etc
I see countless grandparents pushing a child round town in a buggy, looking miserable and not talking to or interacting with the child. It makes me sad to see it.
Try and find a compromise, ask her to continue to do 1 or 2 days a week, saying you value what she is doing for you, but you need more care now and that it isn't fair to her for the various reasons I have mentioned.