Spoke - my heart goes out to you in your family. You have a very tough road ahead. And while I do believe in miracles, from what you've described, the probabilities are not good. I have had a little too much practice dealing with this type of illness in a family member and I would emphasize what some others have said...
Be a good listener, an honest listener for Heartman. People in his situation need someone to talk to that will speak openly about things.
Be a good listener for your sister. Be a strong physical presence for her.
Take care of yourself and ask for help. The hardest thing for me to learn was to accept the offers of help from others and to let them know what I needed. I also learned that this a pay it forward system. The people that help you will not feel like you owe them anything. They will hope that one day you'll be able to help someone in a similar way. But it's your turn now to receive.
Sending wishes for the best possible outcome.
Living life like there's no tomorrow.
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