"Sonny's Blues" is told in the first person from the point of view of an unnamed narrator who, we find out, is Sonny's brother. The narrator in this story is an interesting figure. He's mostly telling us Sonny's story, and this would seem to make him a peripheral character instead of a central one. But this is also his story. "Sonny's Blues" is not just about Sonny's decisions and struggles but also about how they affect the narrator. This story is as much about family and brotherhood and the relationship between these two men as it as about the single character of Sonny.
There are definite advantages to having a first-person central narrator who's telling both his story and Sonny's. Sonny would probably be an unreliable narrator of his own life story. (How clear-headed could we expect someone in the throes of heroin addiction to be?) The narrator can offer us a glimpse of both his own life and of Sonny's.
But there are also definite disadvantages (or complications) associated with a first-person narrator of this sort. We get the story through the filter of the narrator's own memories and biases. Since he is so emotionally involved, we should perhaps question how accurate his recollections are of the events in the story. And when we're dealing with something like drug addiction, we imagine it's very hard for someone to understand what it feels like (and then relay it to an audience) if he himself has never experienced it. So there are perhaps some things we miss with this central narrator.