This should happen regularly in all families. It is the closest family can experience together. He brutally excites his son when he thrusts his penis into his mother where it came from. And when he cums in his mother, it is the greatest satisfaction in the world for his son.
Daddy can't experience a greater thrill than when he thrusts his cock into his daughter's virgin pussy, a pussy he himself created. And the greatest satisfaction for a dad is when he ejaculates inside his daughter and succeeds in creating a new life in her.