Fear Loops
Maybe your dad made him move out ?
That wouldn't remove him from the relationship panel though or the family tree and they should still keep memories relating to him.
It just seems to me as though his character file got corrupted and the game has decided to treat it as though he never existed.
We may never know.
I
think I had a similar situation once but I had been messing about with cheats. I had a single father and two children; a girl and a boy. I messed about with the boy and saved the game. The next time I played the game he had been booted from the family into the SimBin as an adult and the family had no memories of him.