How would you describe each brother?

The eponymous Sonny decides to follow his dreams after he serves in the military. He's less settled and more transient than the narrator but still kind and, as the narrator says, not crazy. He's passionate about music. He was still in school the first time he tried heroin, and it's that which separates him from the narrator. The narrator stayed on the straight and narrow; Sonny veered off the path and into drugs. This was partially because of his attraction to music. The music scene was filled with people doing heroin, and it was expected that he would too. He uses music to express his pain and rage at the world. Sonny also gets emotional about music; when his brother doesn't know popular music artists, he rages at him and then insists he doesn't care what music the narrator listens to.
The narrator is different than Sonny. Rather than express his pain, he worked to overcome it; he wanted to provide a safe and successful life for his family. He follows a traditional path to success, joining the military, getting a college degree, and finding a steady career. He is cared for and respected. He dislikes the crowd that Sonny spends time with and is frustrated by Sonny's repeated drug use. Though he walks away from Sonny for a while, his loyalty eventually wins out and he contacts Sonny again. By the end of the story, he's made strides toward understanding Sonny's passion.

The narrator is a man who has accepted the responsibilities of being an older brother, a husband, and a father. It is arguable that in doing so, he has put his own needs and desires on the back burner. It begins when he promises his mother that he will always look after Sonny after she passes. He joins the military, marries, completes his education, pursues a career as a teacher, and looks after his family. Though he and Sonny go through periods of estrangement, he keeps his promise to his mother and works to understand and connect with his younger brother.
Sonny is a bit of a dreamer, and he is an artist. Like his older brother, he joins the military, but instead of attending college afterward, he seeks to learn all he can about playing jazz through listening, practicing, and becoming a denizen of New York's jazz scene. Unfortunately, that extends to the culture of heroin use that was prominent in the bop era in which Baldwin sets the story. Sonny finds his place in the world through his piano playing and the camaraderie with and understanding of his fellow musicians.