I was a kid when then happened, but I can speak from what my mom might've home through. But I lost my great grandmother, grandfather, and grandmother, within the spam of a year. All 3 were on my mom's side. My great grandmom was 90+ and my grandfather's health was deteriorating, so on some level everyone knew that it was going to happen soon. I guess you just spend as much time with them as possible, and not underestimate how much you will value that time later. When it happens, make sure you have people around you who share your grief. It'll be really sad initially, but with time you'll think about all those cherished memories and it'll get better.