I agree w/Don's posting. If they aren't responding to their father's communications, then you may want to send them one last missive and advise them of the facts and what may happen down the road. If they don't want to communicate w/him, maybe they'll consider touching base w/you along the way. It's never easy to "share" info about someone's health and what very well could happen...but at least they will know that someone cares enough to tell them about their father's health.
Keeping all of you in my thoughts and prayers.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.