Dauofcamom, I am so sorry.
Your questions are very difficult ones and the answers are neither easy to give or maybe even to accept.
Did your mother ever discuss, even casually, end of life options with your dad; or with you, brother and Dad at some point in the past?
If she did that should be your starting point.
Given her otherwise good health (no mets and no other health issues), and what seems to be her choice not to live, she needs to be aware that it will not likely be quick and it will not be easy for her or her family. I won't go into details here unless you wish me to, but I have experienced it. Can I suggest that you find a counsellor that will come to the house? Does her cancer centre offer that kind of support? Is there a priest or minister that can give her guidance?
Don't be afraid to ask for professional guidance/support for yourself, Dad and brother as well.
Again, I am so sorry.