How to help with the depression

My sister was diagnosed with Stage III breast cancer February 2012 will be three years ago, She had a mastectomy and chemo, rarely missed work and thought she had a good attitude.  Age 47. Did not have reconstruction.  Within 18 months the breast cancer metastasized to the bone and presented on the spine in five tumors.She is now at Stage IV.  She underwent extensive chemo which left her disabled with neuropathy in her feet and legs.  At this point she is severly depressed but can not take medication for it as it would interact with the medications she is on for the neuropathy.  She will not go see a psychologist or therapist of any kind.  She won't talk about the cancer. She is mad at the world, never has a kind word to say, etc.  I am at my wits end.  I try to be supportive yet I want to go into a room and hide in a closet and scream.  I can not even imagine what she is going through yet I have finally had the courage to tell her that she has every right to be mad but she does not have the right to be cruel to those around her.  Her feelings are that she is going to tell everyone what she wants no matter if they like it or not after all it doesnt matter she's going to die anyway.   You can't reason with her. I spent Thanksgiving with her and wanted to leave the first night I was with her.  I barely made it through the four days I stayed with her.  I'm lost....How do you help someone that is going through cancer and is that depressed. She feels that I'm all she has yet it's all I can do to be around her and I get angry with myself because I feel that is selfish on my part as she is the one that is sick and I'm not. She has completed another round of chemo the tumors are gone for now. I realize that it is not a matter of if the cancer will return but when. Yet my feelings are that even though there are complications with the neuropathy, etc. that she is alive.  She does have family that has suppsorted her and love her yet I feel like you get a verbal and mental beating everytime you are around her. Help me to understand what I can do to make her feel better somewhat about things.