I'm sorry to hear about your mum, i know the agony you are going through as its not so long since i went through the same thing. I chose to believe that my partner hung on so long in order to give me time to accept the inevitable outcome.
At this point the suffering is more yours than your mums, and all you can do is be there for her. That might not be any comfort to you, but i do believe that she will know you are there.
I hope you find the strength to get through this, as your mum needs you to be strong.
I can imagine how you are feeling, it is agony for those watching, isn't it. Keep talking to mum, I'm sure she is, in one way or another, aware of your presence and that she is surrounded by love. She will, at some point, feel able to let go. It's different from person to person, some fight for longer, others slip away more quickly. Which doesn't make it any easier.
Sending you strength and courage and wishing you and your mum all the best on this difficult journey.