Do you mean the Gambino brothers, Tommy and Joseph?
Tommy was a capo, running the garment rackets for his father. Joseph had a trash hauling racket. Both went to prison for racketeering.
Neither was considered forceful enough to become the head of the family. What's more, they were loyal to their cousin, Paul Castellano. They didn't protest when he was made Don, nor did they make a move on Gotti.
They really don't make a good comparison with Fredo. I don't think either of them was "passed over," since apparently they never wanted to be Dons.