My grandmother is dying of cancer at the moment. I deal with it by staying busy (I'm committed to so many different things that I have absolutely no free time, and probably won't until she's passed on). It works for me.
If there's nothing that you can do to help, then I suggest make yourself busy. If you like video games, then start playing one on a higher difficulty level. Anything - work, recreation, art - that will keep you busy will help in the short term. Try not to brood over it. I mean, it's natural and healthy to be sad/worried, but it's not good to sit and worry about it all the time.