The relationship between Sonny and his brother is extremely complex, to say the least. Baldwin designs these characters to act as character foils; therefore, accentuating each's character by contrasting it with the other.
Sonny is a free spirit, restless and dreaming, he seems to have wandered through life in search of something he is not sure of. He has battled addiction and isolation. He is the younger brother and desperately wants the narrators approval.
Sonny's older brother is very conservative, unlike Sonny. He does not take chances. He has tried to take care of Sonny as much as he can but in the process has forgotten to support Sonny in what Sonny wants. Due to the untimely death of their father, the narrator takes on the role of authority in Sonny's life.
It is not until the end that the narrator truly "hears" Sonny. When Sonny plays his music, the narrator finally hears his story and it all makes sense because he finally stepped into Sonny's world instead of always waiting for Sonny to step into his.