When the narrator last saw his mother, he was told about a long-dead uncle he was never aware existed. This man was his father's brother, and like Sonny, he had a strong passion for music. For a long time, the narrator's mother did not want to bring up the subject in front of either him or Sonny, but she broke that rule to warn the narrator to watch out for his brother.
The uncle was killed by a group of inebriated white men. They were driving their car fast and would not slow it down either to avoid hitting Sonny's uncle or to see if he was okay after the impact. The uncle was killed instantly, his remains a mess. This event not only ended the uncle's life, but traumatized Sonny's father forever after, embittering him immensely.
This anecdote is meant to strengthen the narrator's concern for Sonny. The uncle is almost like an earlier version of Sonny, from his musical interests to his being "a little full of the devil." Sonny's mother feared that he might end up like the uncle if he continued living the way he did, and she hoped the narrator's loyalty would prevent him from going into a downward spiral. For the narrator, this is a difficult feat, even with the example of his deceased uncle looming large.