I'm so sorry about your mom, my heart aches for you and your family. When a family is hit with this sort of tragedy, it's so difficult to truly know what to say and do.
Christi gave you some good info.
Continue to talk to Mom, be there for her. Talk to her, let her know how much you love her.
Perhaps you can speak to a priest, a minister -- does your mom belong to a church? When we knew that my sister was dying (not from cancer) I was able to talk to our parish priest from time to time, he was so very compassionate.
My sister passed away in January of this year.
Again, I'm so sorry...
Vida