I think you're going to do just great in a very difficult situation.
I agree with what Avishai said; be mindful that you are participating in a "theatrical spectacle" for the younger members of your family - you are going to show them how a rational, objective, loving person behaves. They can witness insane robot behavior at the Kingdom Hall.
You know that you control how you act: you can choose to be cordial to everyone, even when they are rude and stupid.
Thank your father for whatever good he did for you in his life. I get the impression that your relationship was not too antagonistic, so there must be some memories of good times. Keep in mind that not all fathers stick around to see to it that the young ones get fed, schooled, etc. Your Dad, faithful to his wife and children, is a member of a dying breed - no morbid pun intended.
Let him know that YOU will remember him, and let him know if you will miss him. You might ask him, when you're alone with him, if there's something he'd like you to do for him after he's gone (that doesn't involve the WTB&TS).
If your father is in pain now, that pain will certainly cease when he dies. The valley of the shadow of death was a quiet place the couple of times I personally have been there. There is nothing to fear.
It is also true that there is nothing to look forward to, but you know that.