What difficult time this most be for you and your family. You may want to speak to a social worker at the hospital/center where your mother is being treated to go over and understand the options better. What does you mother want, that is a big factor - I myself would say quality over quantity. I remember having to make decisions during my parents final illness (Alzheimer's and Congestive Heart Failure), based on what we knew of my parents, we choose palliative/hospice care , and not aggressive treatment.
You may also want to get a 2nd opinion to confirm your mom's prognosis, another set of eyes and experiences can only help you make the right decision