Don't worry Neptune, I feel you. I have the very same kind of experience. One monday, my father went to see his father, and my mother had her phone open in a call with him, on loud speaker mode. I overheard my grandfather's voice, as if he was weakening... the next day, when I came back from school, my fears were justified: he had passed. We visited my grandfather shortly after that, and it was heartbreaking to see her cry. I was, like you, worried about her continuation... sure enough, two years later, she got her turn.
May your grandmother rest in peace, man. The pain of losing someone this way. I hope your grandfather manages to live through, and hopefully he has a long time to live left.