I would only tell them if they wanted to know.. and if they didn't and I could see they were having problems with lung cancer or something, I would try to give them a hint like you need to go have a chest x-ray done. Or you should get your lungs checked out.. or something along those lines.

I don't think I would really want to see my families future, but I guess I would probably no more or less inclined to tell family and friends versus a random person.