I know exactly how you feel. When my younger brother was diagnosed, I cried like a baby. I felt helpless, useless, like there was nothing I could do to make it better somehow. Just think of all the good things, and make good memories when you can. There may be hope yet, all of you just have to stay strong.
Also, to keep in mind, she may simply need quiet time to herself. Facilitate that for her. There might be others who want to spend time with her- put her needs first. Whatever you are feeling now, she is undoubtedly feeling the same exponentially. If she needs time alone, give it to her. Don't ask why, or complain, just say 'okay'. It might hurt, but she is probably asking this for a reason.
Don't treat her like an invalid, don't treat her as if she is made of glass. My brother hated that. He hated how everyone would treat him like he was so fragile- he wanted to do stuff on his own, just to prove to himself that he could.
When she does need someone to talk to, she'll ask. Just let her know that you're available for her. All of you are undoubtedly under enough stress. Stress makes people do crazy things, and sometimes say things they don't mean. In such a situation, they're exhausted, sad, angry, and a billion other things. Try to make things as smooth as possible for everyone. Try to avoid arguing, especcially around your Mother.
Every cancer patient is different. Different health issues, different circumstances, so many different variables that the outcome can vary wildly. In my own experience, I hoped for the best, but prepared for the worst. It's very hard, but the sooner you realise that all you can do is simply help make things easier for your mother, the better off you will be. Accept that, and accept her wishes, whatever they may be. I wish you the very best for your family.
-Hecateslover