It could be that Tom Willis, the father of George Jefferson's daughter-in-law, had a brother who was a captain in the 'N.Y.P.D.' tactical squad. But just because they looked alike doesn't mean they had to be identical twins. There could have been a few years between them.
As seen in the episode "Fast Gun"
He shows up later in an episode entitled "The Patriots"
Franklin Cover played Tom Willis and the Captain......