The last sentence of this excellent short story is actually an allusion to The Bible and is used to denote the way in which Sonny has made peace and gained understanding both with himself and with his brother. After a story that talks about the various misunderstandings and animosity that has existed between Sonny and his brother, it is only at the end, when Sonny takes his brother to hear him play jazz so that he can understand how important it is to him, that the narrator makes peace with him, as symbolised when the narrator buys drinks for his brother and the other musicians on stage. Note how the story ends:
For me, then, as they began to play again, it glowed and shook above my brother's head like the very cup of trembling.
This is a reference to Isaiah 51:22 where is says, "I have taken out of thine hand the cup of trembling... thou shalt no more drink it again." Thus this allusion signifies how Sonny has overcome his various problems and sufferings and found release through his music, and likewise how his brother has made his own peace with Sonny and how attached he is to jazz music.