As a cancer patient myself, one whom the doctors offered no hope of survival because of the severity of it when it was diagnosed, I was inundated by people that felt they knew what was best (or rather easiest) for me. My own family got angry when I refused to lay down and accept that death was the only option. One member overstepped his bounds and proceded to try to get me Hospice Home Care despite my protests. He was told that unless I had accepted death as the inevitable Hospice would not even consider me as a client.
I had not accepted that death would come within two months, I had no intention of laying down to die and was hurt and felt alone when that was not what everyone else thought would be "best" for me.
Stand by your mother, support her with all your heart, this won't be easy, it is a very emotionally traumatizing thing to bear witness too, much less take part in. There will be days when you no longer think you can cope with it, there will be days of joy, a certainty that your mother will win the fight. No one knows exactly what the outcome will be.
Tell your concerned relatives that your Mother is quite capable of making her own decisions about wether or not to undergo treatment and that while their intentions may be the best, they are hurtful to your mother, they serve to further discourage her and leave her feeling unwanted, worthless, a burden. Unless your mother wishes to discuss the option of Hospice ask them to please not discuss it with her as she is not receptive to it and it only serves to upset her.
Your mother has the right to fight for her life, the decision is hers and having you there to support her and care for her means so much more to her than you will ever realize. Ask them if they would lay down and die so willingly if they were the patient or would they try to fight!