Mama tells the narrator about his uncle because she wants him to know "It ain't only the bad ones, nor yet the dumb ones that gets sucked under" in their environment. She tells of the unintentional, but, nevertheless, racially-motivated harassment of her innocent, fun-loving brother-in-law that leads to his killing so that the narrator will understand there are circumstances that often place black people in dangerous and unprovoked danger that can easily lead to death.
"I ain't telling you all this," she said, "to make you scared or bitter or to make you hate nobody. I'm telling you this because you got a brother. and the world ain't changed."
Mama wants her older son to realize that Sonny needs watching, too. Like the fun-loving brother who was too unaware of the cold realities around him, Sonny, also, is vulnerable because he possesses a similar temperament since he is a musician as well. That he is of a weaker nature is apparent because of his use of drugs. So, the narrator promises his mother, "Don't you worry....I won't let nothing happen to Sonny."