At 91 I'm pretty certain that standard treatments of chemicals, radiation will kill her. In fact, her last days, she will most probably beg to die.
I cared for my mother and when I found out her case was terminal, I made sure that we kept postponing those poisons being put into her and her last months were as pleasant as anyone with metastatic cancer can be. She enjoyed nice Thanksgiving and Christmas dinners and went to the hospital two days after Xmas and passed away less than a month later in hospice care.
Palliative care (aka Hospice) is what you want to give a 91 year old, particularly with congestive heart failure.
I'll pray for your mother and you and your family with the hopes that you can all come together and make her last days pleasantly memorable and comfortable for her and for you and all the family.