Please read and re-read what Steve2 had to say. Your family rejected you years ago and continue to do so by their silence about your father's illness.
On a personal note, I've had limited phone contact relating to necessary family medical history with my mother who has shunned me for 27 years. She took the opportunity during one of these brief few moments to defend her husband for attempting to rape me when I was 12 years old.
My mother now has cancer and is rejecting medically advised treatment. And, no, I will not attend her funeral when the time comes, because she is a stranger to me.
No good can come from communicating with cult members. They are manipulative predators, who if permitted, will use your kids as the next generation of victims. Write a letter to your father if it makes you feel better. But for the love of all that is holy, stay away from these twisted people.