Why does the narrator feel a sense of responsibility toward Sonny?

You understand that the narrator is Sonny's brother. Here is a link to a study guide on Sonny's Blues: Sonny's Blues Study Guide by James Baldwin
Some essays: Sonny's Blues Essays - Brothers' Relationship in Baldwin's Sonny's Blues Sonny's Blues Essays - An Analysis of Baldwin's, Sonny's Blues
An article: http://everything2.com/index.pl?node=Sonny's%20Blues