Ah, family. They can make us crazy and angry; they can love us and drive us away at the same time. This is precisely what happens in "Sonny's Blues." The two brothers in this story clearly love each other, but they don't understand each other and they don't condone each other's lives. It's precisely because they love each other so much that they have the power to really hurt each other. But they also illustrate how strong the ties are that bind them, because in the end they find their way back to each other and reaffirm how much they need each other. It's a hard road they travel to get to this point, but they do get there.
Family operates on another level as well in the sense that some characters create their own family separate from the one they're born into. The narrator creates a family with his wife and children (this is the more traditional family), and Sonny creates a family with his fellow musicians. Although the narrator doesn't get this, Sonny is able to surround himself with people who love the same things he loves. But these same people, this same family, are also a negative influence, since some of them fuel Sonny's drug problem.
No matter how frustrating family can be, it's family that gets Sonny back on track.
Sonny's family is partly why he turns to drugs and is, therefore, an unintentionally negative force in his life.