In "Sonny's Blues," how does the mother's tale about the narrator's father and father's brother shed light on this story?

In one segment of a memory flashback in "Sonny's Blues," Sonny's brother recalls a conversation with their mother during which she tells a family secret about their uncle, their father's brother. The narrator is seven years older than Sonny and though they were close in the darkness of suffering caused by racism and the darkness of escape from suffering through movies, he feels the separation caused by the seven years; at one point he worries about whether those years can ever be bridged. Their mother reveals a tragedy that affect their father and his brother.

The narrator learns that their father's brother, their uncle, was shot down by drunken white men. Their father lost his brother to the wild brutality of racism. Their mother is make an earnest plea by way of an object lesson that the older brother, the narrator, promise to always take care of Sonny because brothers are important and, at times, all the help that each has.

This family secret, kept from the boys in an attempt to reduce their suffering, sets up the  narrator's conflict because he doesn't know how to help Sonny and has given up on him. It also points out that suffering comes, as is said above, from wild racist brutality, the life darkening brutality of racism. Since this is so, the brothers need to appreciate each other and stand by each other and the elder must promise to help the younger to prevent them both from being drowned or shot down by racism.