To borrow a phrase from our modern society: It's complicated.
The narrator does have his brother's best interests at heart. He wants to see Sonny do well and be happy—and he thinks that he, as the older brother, is more knowledgeable about how to ensure that than Sonny is himself.
The narrator has taken a more traditional path through life. He has become a teacher, gotten married, and had several children. His brother had one dream that he shared with the narrator when he was still a kid: He wanted to become a musician. The narrator had scoffed at Sonny and tried to set him on a trajectory more like his own. As a result, Sonny floundered, trying to follow a heart that was not his own. He was forced to live with his brother's in-laws while his brother served in the military, and was told that his music wasn't appreciated there. Eventually, Sonny turned to drugs, which was what he had foreseen as his future, even when he told his brother that he really wanted to escape Harlem.
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