I get the feeling Dave is an older brother. It fits more into the character of Sheppard and how his father rejected him, his older brother had to take up the mantle of the family

It does make more sense that way, but then Dave said that John left and Dave had to stay and be Dad's right hand man, which made me think that John was the oldest and bailed, leaving younger Dave with no choice but to fulfill John's duties as first son.