Oh, I'm so sorry to hear this. I'll be thinking of you and your family.
Regarding lost parents, I lost my dad to cancer when I was 23. I certainly wasn't prepared for it, and I still miss him terribly, but life does go on. The first year was hard, and I did my best to stay busy, and it got easier over time. The experience definitely matured me in ways that I may not have matured otherwise, and I am sort of thankful for it, in a weird way. Most of all, I'm just thankful that I knew my dad at all, as some people aren't so lucky. I'm so sorry to hear that your niece and nephews will have to continue to grow up without their father, but hopefully the experience will at least make them stronger. It tends to put things into perspective, as once you have faced something so horrible as losing someone you love, the little things just don't get to you as much as they used to. At least they will still have their mother, while many others have lost both parents.
I'm not trying to diminish your grief or feelings of sadness by any means at all. It is a terribly sad and frightening thing to face. But, I tend to find that at times like these, I think of people who have it worse than I do, and it helps me go on. It was really the only thing that helped me face losing my dad. I kept thinking of my cousin who had never met his father, and of others I knew who had lost someone close to them, and in a way, it helped me be grateful for the time I had with him, rather than lament the time with him I was going to lose. Hugs and love to you and all of your family in this terrible time. Just do the best you can to be there for them.