When we go check on her and find out why she's crying, she says that she is going to miss us so much when we get old and die.
Grow old and die? Geeze, your mom must not have much faith that Armageddon is 'right around the corner' if she's telling your daughter that you're going to grow old in this System.
My daughter nearly floors me when she said that grandma told her that if you make Jehovah happy you never die.
I don't know how old your daughter is, but I would say something like "Grandma's religion has been telling people that for over 130 years and they all died."
Your situation is not uncommon and you may just have to put your foot down and tell your mother you do NOT want her telling your daughter things like this because it's upsetting to the entire family. If she persists, you may just have to curtail the visits.