My thoughts go out to you and your family, espc your mum. Time is precious so make sure you all spend time together as much as you can.
My mum died over 9 years ago, aged 48, of Ovarian Cancer which is a very hard thing to take even now as I still miss her at certain times.
As the others have said tell her how much you love her and just be there for her as much as you can and if she is scared or wants to cry then be with her.
I know its hard and you feel like a failure not being able to do anything but just being there is very important to you, your family and esp your mum.
Send her my very best wishes and love to you and family. Take care and be strong. May god be with you all and watch over you.
Lots of love