S2·E20 Enemies and Brothers

Laredo's new lady doctor is Captain Parmalee's sister-in-law, widow of his brother Frank who died in the Civil War. Reese, Chad, Joe and Erik are sent after outlaw Bart Cutler. They soon learn that Cutler is actually Frank Parmalee and, although they try to keep the Captain from finding out, developments eventually lead to a showdown between the Parmalee brothers.