I agree with Existentialist, not many cancers are heredatory, and it really depends on the type of cancer too. I'm sure your father will be fine, you have to trust that the doctors know what they are doing and that they will only do what's best for you father, and if they say that they think your father will be fine then, like I said i'm sure he will be. It's also good for your father that he has loving family around him and that there is nothing major (from what you have told us) for him to stress about.
Thinking that you might be in line to get cancer, after seeing people that you are related to get it is only natural, I know that when both of my grandfathers had triple bypass surgery for heart attacks I was terrified that I would have a heart attack when I was older (not really the same thing but it's the only thing I have to relate, there has never been cancer in my family, touch wood) but the best thing to prevent cancer is to lead a healthy life, few fatty foods and exercise, and by doing that and keeping your body healthy the only reason for getting cancer is if that's what was in store for you and there is nothing anyone can do about it. But be optimistic and try not to think about yourself getting it rather put all that extra energy into supporting your father. Good luck to you and you family and I believe everything will turn out for the best.