Thinking of you, SueDonim and Old Fossil. My DD lost her (good, loving and responsible) husband to cancer 2 years ago. He had been seriously ill for several years and she had to stop work to become a 24 hour carer for the last few months of his life.
My GC was still at primary school when SIL died. GC has no siblings as parents turned out to have fertility problems; and of course when the illness became terminal there were other priorities.
I was being treated for cancer myself during the last few months of my SIL's life; lived at a distance and could be of little help at that time. As I am still here,this last I hope will change in the near future.
Worrying on its own never makes any difference. Talking things through can help.Learning to manage anxiety can, too. So can counselling from empathetic professionals. And practical and emotional support from friends, family and neighbours is without price.
I mention especially the wonderful staff at the hospice, who ensured the little family had contact with another going through the same thing. The school who assigned a teacher to give an hour of her time ( 1-1) to my GC every week for months so feelings could safely be expressed in private. And my DS - who has no children of his own and who also lives at a distance- but who made sure he was there for his sister and travelled to be with her and GC at the last. And who took several days off work to be there when GC started secondary school six months later.
Don't be afraid to ask for what you need - reassurance, advice, information- whatever. Most people in my experience are glad to help if they can.