Sarty's brother doesn't get much of a role in "Barn Burning." Until his brother speaks his first and only lines in the story, "Better tie [Sarty] up to the bedpost," all we know is that the older brother chews tobacco constantly and that he has his own bed. After these lines, we know that his brother has accepted his role as Abner's accomplice. This would put him at odds with Sarty. Still, we aren't ever told how Sarty feels about his brother.
We notice that on several occasions Sarty thinks someone is talking to his brother when they are actually talking to him. This suggests that he's being asked to perform duties previously assigned to his brother. Could this make Sarty feel like he's turning into a version of his brother?
The brother seems like a mere sketch here, a sign of what Sarty could become if he doesn't get away. In the novel, Snopes: The Hamlet, The Town, The Mansion, which Faulkner wrote after "Barn Burning," the older brother gets a starring role, and even a name (Flem, to be exact). After a series of ruthless moves, Flem becomes a wealthy landowner and gets a mansion of his own. We wonder what Sarty would think of that.