The two older brothers of Eben are mainly used in this text as foils for their younger brother. Let us remember that whilst all three brothers share a hatred of their father, Ephraim, it is only Eben who choses to act on his feelings and try to work to secure his father's downfall. The older brothers, although they do genuinely hate Ephraim for what he did to them in their lives, are more than happy to wash their hands of Ephraim and his influence on their lives and walk away.
This is something that Eben exploits to ensure that he is left alone to pursue his private agenda. Eben's offer to them of gold in exchange for renouncing the farm and Simeon and Peter's quick acceptance of this offer implicitly compares them with Eben in relation to their ability to not let their adult lives be dominated and ruined by their father. They are more than happy to seek a new life in California, leaving their father behind. Eben's inability to do this reinforces the extent to which he is obsessed with his father and gaining revenge on him to an unhealthy degree.