Dave Sheppard was Lt. Colonel John Sheppard's estranged brother. He worked with their father Patrick Sheppard, a utilities mogul, and presumably took charge of the family business(es) after Patrick died from a heart attack.
John hadn't seen Dave since he joined the Atlantis expedition, but they met again at their father's wake in 2008. Dave believed that John was there to challenge the will. Dave informed John that their father had always regretted their estrangement over John's choice to join the Air Force. Eventually the brothers reconciled, with Dave inviting John into his house to talk. (SGA: "Outcast")