Hurt&Confused-
First of all, let me introduce myself. I am the wife of a cancer patient. Actually, my husband is in remission now. We first found out about his cancer right after our first anniversary in 2003- and were absolutely devastated. He had bone cancer of the left femur. Neither of us had a history of cancer in our family and so we both thought we were in the clear. Just goes to show that cancer doesn't care who you are. We both struggled with the numerous rounds of chemo, biopsies, surgery (limb salvaging surgery- replaced half of his femur with a metal rod)and the strain this put on our marriage. We were so newly married that his family felt it was their duty to become so intimately involved- which was very hard on me at times. There were times that I just wanted to tell them to go away because this was between he and I. Even though there were many times where I appreciated their help- I felt like an outsider.
As for your situation-- I obviously don't know your girlfriend or her personality or "where" she is coming from on her decision. It could be that she knows that her mother doesn't have long and she wants to spend as much time with her- alone- as possible. It could also be that she doesn't want you to see her mother in this kind of shape. Cancer is an ugly, ugly disease and many people would rather hide- due to their appearance- than see anybody. I know that when I was hurting- I said mean things to those I was closest to. I was angry that this disease could happen to US- and even though I knew my family and friends were trying to help- I had to let some of my feelings out at times- sometimes on to them. But, ultimately, I still wanted them to be there when I needed them. There were also times when people would say "I am here to help- just let me know what I can do." I heard that a lot- and appreciated it. But, when it came down to it- it took too much energy to THINK of a task that that person could help with- it was quicker to just do it myself. Furthermore, I never wanted to bother them. There were times that I just wished they would KNOW what I needed and just do it without being asked. We had some friends that made us lasagna and froze it in individual containers for us- that way we didn't have to worry about dinner when we needed it. That helped a lot. Perhaps thats something you could do for her. I'm sure she wouldn't turn that down.
I think that if you do little things- things that allow you to not become TOO involved with in the actual caregiving of her mother- she will see how much you are helping- and maybe she will begin to open up to you and let you in. You will also feel better for helping. Think of things you know they like- compile songs for them on a cd, rent them some movies to watch, or make them dinner. These are all things you can do from the "outside"- and they can enjoy them whenever they want.
Just be patient and continue to respect your girlfriend's space. She may not know what she wants or needs- but being a steady friend is a really big gift! As hard as it is- you have to put their requests first.