Do I tell dad his prognosis is less than 6 months?

Your siblings may be afraid of their own coping skills. I agree with AlwaysMyDuty. Your Dad should be able to have discussions, express things to people you all may not know about. He may have insight, as stated, and he is protecting others from all the emotions! Having closure on one's own life is a given! God Bless You All:) xo

Thank you all so much. I've been reading some here and I've already learned so much. What a blessing this site is.

Dpeek68, yes, I would tell him and best wishes with that. Sending you hugs(((((hugs))))). This is a good place to come for emotional support. I hope you return throughout this hard time for you and your Dad and reach out if you need to vent. Blessings. ~Mishka

Here's the thing - most people kind of know - and if no one will "tell" them, it means that it is too terrible to talk about. So they have to suffer in isolation in addition to everything else. This is hard, and with some exceptions, keeping secrets would make it even harder. Bless all of you at this difficult time!

Thank you. I know that's the right thing to do.

Unless your father has advanced dementia and unable to comprehend anything, it is right to know what lies ahead for him. Back in 1968, my grandfather was diagnosed with colon cancer and nobody told him. I've always thought it was a horrible, disrespectful thing to do. It is his life. He should have the opportunity to get his affairs in order. What if he has a "bucket list"? Would your siblings want to be treated this way?
My sister-in-law has stage 4 cancer. She has never been given a time frame but she knows she's terminal at least. She got her affairs in order. And if she ever has a day she felt good enough she would get to those things on her bucket list.