I am sorry to hear your family is going through this. Have you all tried MD Anderson? My father had Chronic Lymphatic Leaukemia and it was the best place and most aggressive for difficult cases like his.
My father had 6 siblings, 5 sisters and 1 brother. His disease made them uncomfortable because their father died of the same thing a year before my father was diagnosed (I was 6 months old when he was diagnosed). They became closed off, stopped talking to him, they were just uncomfortable and didn't know how to deal with it, it sounds like they had a lot of the same feelings as your husband did, they ignored it thinking it would just go away. Now, one sister did donate bone marrow once for a transplant, but some of her sisters got made at her for doing it saying she was risking her life (not true).
So, for his second bone marrow transplant he had to do a few years later, I believe the doctors used his own bone-marrow since no one else would donate, sterilized it somehow and then reinserted it. It was something to that effect.
Anyways, all my dad ever wanted was support from his family. They visited him a few times in the 25 years of his disease (though the last 10 years he was half dead anyways), but were still very reluctant to visit and acted like they didn't know what to say, and his own mother pretty much abandoned our family, I suppose because we reminded her of her husband's death. All they had to do was act normal, joke around talk about the weather, their kids, sports, religion, the news, hobbies... whatever, but they didn't. Of course, when he finally died, they all came to the funeral and sobbed and said they regretted not being there for him and letting time go by.
Anyways, that is my story. All people with cancer want is support and love and some normalcy in their lives. Family can provide that because a lot of friends will run away out of fear too. I am sure it is rough for your husband to go through this as well, but if he doesn't come to grip with the reality of the situation, all he will have after his sister's passing is guilt and bad memories. Just be there to support him and urge him on for her, since she is the one that needs the strength right now. Hospitals have on site grief and family counselors that can help give words of comfort and advice as well. Your husband's attitude is a very common form of grief, but it can be quite difficult if not handled correctly.
Best of luck to them!