As others says it's too painful for them to acknowledge. The word cancer strikes fear into the heart of everyone, in fact my mother in law couldn't even bring herself to say the word. In this day and age the prognosis is very good. My sister in law has had cancer for the last twenty years, since the age of 40, all over her body and still she lives life to the full. Speak to them about it, email your daughter telling her where you are with it now, if you speak to your son try saying lightheartedly, 'honestly you always change the subject'. Until they know you're slightly upset by their attitude they're not going to change. Try to be positive, difficult though it must be, when you speak to them, you being confident will at least give them some comfort.
Anger management!!! Help needed.