@trying2learn,
Like Edgar I've tried to keep this thread at arms length. My wife died from breast cancer in 2004, so it does bring back memories, which have been more acute of late because her mother has just died, and I'm going through the same procedure.
I'm sorry to hear about your sister, if nothing else the chemo will extend her life. I am a great believer in healthy body, healthy mind. As long as there is life there is hope, even if it's a faint hope. Sometimes cancer can go into remission even when it seems hopeless, and I think the state of mind has a lot to do with it. Unfortunately my wife had a lousy childhood and low self-esteem, if she had been happier things may have turned out differently.
I wish you and her all the best, try to keep positive, it can't do any harm. You can be both optimistic and realistic at the same time. Chin up.