I'm sorry to har that your family is going through that. I think the best you can do is tell your mom that you are very sorry that your grandmother is in that situation. That you love her and that no matter what happens you'll be by her side every step of the way. Assure her that if she needs to talk, cry or anything you'll be there for her.
Even if you say she may have little time to live. I'll have your family in my prayers. Hopefully, she beats the odds.
I don't have the words to tell you what you should say to your mom. I just lost my Mom in December, & I know no words anyone could say made me feel better. It did, however, help to just have people around. When I was alone, that's when I broke down the worst.
Just try to be with her if you can (when she gets back or whatever..) Also, don't worry that you aren't crying or really grieving yet. I honestly can't remember who of my family took my mom's death the hardest because I was drowning in my own pain & emotions so much. The only other person I paid attention to really was my brother but that's because I was trying to make sure he was okay too.'
Once again, I'm so sorry. I hope you guys get a miracle.