honestly dude i dont know what to say to this question, i feel your pain, but i would let your mom live as long as she can if that is her desire, and not try to change her mind on that, sure its sad to see her suffer and live miserably but think about it would you want to have your child see you die or would you want to stay with him as long as possible no matter what happens to you?
now im not trying to make your family do as i say or anything in that nature but i am saying that they need to respect your mothers wishes and make her life no matter how short the best possible till that horrible day comes.