Howdy, I'm sorry that your grandfather is very sick. Unfortunately, sickness and passing on is just part of the cycle of life. Death is something that is SO confusing, and no one really has accurate answers.
It's perfectly normal to feel very empty when you first realize someone is dying or has passed away. I've felt the same way when my neighbor and my classmate passed away this past year. I hope you don't feel guilty for not crying because there's nothing to feel bad about. Everyone deals with pain and grief differently. Some people just feel numb to it all together, which is probably how you feel, right?
Numbness is usually brought on by disbelief, shock, or because one doesn't want to think about the pain of a situation. You'll get through the numb feeling, though, by talking to your friends and family about the situation. Then, you'll start to acknowledge that your grandpa has a serious form of cancer. At that point, you'll probably feel very sad, but this is also a normal feeling.
The only way to get through the misery of serious illnesses of loved ones is to stand by your family and continue to support each other. I'm sure your mom and grandmother will be sad, but they've had people in their lives pass away before, certainly. You, on the other hand, are much younger than them, and it will be more confusing for you. I don't know if people in your life have passed on before, but it is a very disturbing feeling. I can tell that you sense that feeling based on your overall confusion.
As for how to help your mom and grandmother, you can just be there for them if they need someone to talk to, or just to listen to them talk.
I hope the best for your grandfather and the rest of your family, including yourself. PM me if you need anything, anything else at all.
Good luck,
Rachel