I'm so sorry your family is going through this, she sounds like a strong and wonderful woman who is still trying to go forward with her life.
My father had Leukemia and I grew up with these same fears, we were told about 13 different times to go home and plan a funeral. He died when I was about 22, so he lived much longer than any of us thought. As an adult, I remain safely detached to my husband should I ever lose him and have to be the sole provider and without a partner for my family. It kind of goes with the territory I suppose of growing up knowing that early death is inevitable. My friend just lost her 14 year old son to cystic fibrosis, again, another disease where young death was inevitable.
All you can really do is love deeply and make great lasting memories with her. Treat her like she is normal... a lot of people have a hard time with this. My father had 6 brothers and sisters and most of them ignored him b/c they were too scared to get too close knowing he would die and plain "didn't know what to say".
Make her burden light, but if you need to cry it out and talk with friends or a counselor about it, then maybe that will be helpful to you.
My father, despite his illness, was a ray of light in a lot of people's lives. He endured through the pain and disfigurement and remained comical, joyful, thankful to God, a volunteer in the community. There is always hope and there is always the spirit that keeps us all united.
I believe, that after death, family units will remain eternal and that God has a purpose for each of us. That great things can come from tragedy. I believe my father's life of illness and death made our family very close and loving, because we knew we had a short time on this earth together. Several people have told me how his example changed their lives for the better.
Have you heard of Stephenie Nielson? She is an amazing mom of 4 and plane crash survivor, who now endures a lot of pain and struggles. She has touched many and s brief video of her is here. It is very uplifting and may help bring you some peace:
http://www.youtube.com/watch?v=KHDvxPjsm8E
She has a wonderful blog here that is always so inspiring:
http://nieniedialogues.blogspot.com/