Both of my granddad's died before I was even born, but both my nan and grandma are still alive and it's been around 18/19 years. She'll be devestated yes but that doesnt necessarily mean she'll be next.
My uncle died 2 years ago, and we all knew it was going to happen (he was an alcoholic, only 50). I'd only known him really for 18 months, as we didn't really keep in touch before he was first admitted to hospital (and that was only because he told the nurses he wanted my mum (his sister)there). Every chance I had I would go and see him in hospital, one reason because no-one else seemed to visit him and two because I hardly knew him and wanted to get to know him before he did die. The thing I'm trying to get at here is that even though you're bound to be upset, you gotta put on a brave face and treasure the time he has got left. As Nix! said, mourn after he's gone. You know about it, so you can use the time left to spend time with him and so have memories of happy times. If it was unexpected, it would be worse because you wouldnt have the time to spend time with him.
I know it's hard, but you've gotta try and make the best of a bad situation. There's no written way of how to cope, people cope in different ways. It sounds like you're a very close knit family, and so you'll be able to help each other, you'll be able to talk about all the good times you each had with him. With me, the rest of the family (my mums/uncles brothers and sisters), we're not that close at all, hardly see them, never ever see or speak to my grandma, and so didn't have that support.
Not sure if that'll help, but I'm around if you want to talk or anything.