Now that my dad has told us, he wants everything to go back to normal. My sister has cut off communication with him; I have remained somewhat civil. But how do we explain to our father how badly this has hurt us and that we aren't going to be the same happy family we were?

Well, I don't even see that the father is asking his children to accept their half brother. He is just asking for the same relationship that he had before he told them that they have a teenage half brother.

It takes a big person to forgive another person's transgressions and it seems like these adults cannot see beyond themselves.