I am in the middle of some legal action regarding my mothers/grandmothers estates. Its a major f'ed up pain in the ass.
You need to get Grandma to state exactly what she desires to the entire (cooperative part) of the family. Have it documented and notarized have it looked over by a lawyer to make sure all her ducks are in a row. Exactly what she desires as the outcome of her estate while she is still healthy and metally with it.
For her safety, check in with her as much as you can and she is comfortable with. In retrospect a lot of communication with my mother and grandmother would have prevented this legal action. My mothers death was unexcpected though and I was young, so I didn't understand the ramifications. You are in a different situation though and can take proactive action.