Much as I'd love to hand you a baseball bat and say "Go get 'em, tiger." I can't do much more than offer you my condolences and wish you and your loved ones strength.
The best advice I can muster is try to focus on what you can do rather than on what you can't. I know the distance sucks, god do I know it sucks, but if you can, try to give them call? Maybe hit up skype for a video-call? Write a letter? None of that is going to kill off cancer, but any type of distraction is a godsend. Tell them how things are going for you. Share a good show or send them a good book. It may seem trivial, especially in the face of such an overwhelming situation, but small things can make a difference. We can transmit our feelings in many different ways. It may seem counter-intuitive, but people fighting day in and day out can really do with something else to talk about. While it's really shitty you can't physically be there for them, it does put you in a position to fulfil that kind of role.
Still, best of luck. Shit like this ain't easy.