I feel for you man. It's not easy knowing how a loved on will end up. I lost one of my closest friends, considered him my brother, to cancer. (He was 18) It got to the point where I couldn't bear to see him but I still was by his side cuz it made him happy to see his friends. Unfortunately, the best thing to do is to slowly accept their fate, and make them as happy as they can be before they pass. But remember, once they go, all their pain will end as they move on to a better place.
Sorry to hear about the news. Just remember, bad times are always followed by better days. Days that will make you happy, and joyous.