Judge Wendy Joseph said it had been said during the trial that Dean Smith had a loaded gun and was "going to shoot someone".
Cyreena McKoy said the murder had devastated her family. Her brother Lee was "like a lost soul" without his twin.
"Dean Smith had no reason to commit this senseless act," she said in a statement.
The court heard that Mr McKoy became the innocent victim of a long-running feud between two gangs.
He had had friends who were associated with the Holly Street Boys, while Smith was a long-standing member of the rival Lordship Lane Boys.
He arrived with a group at the Clerkenwell House wine bar in central London as Mr McKoy and his friends were peacefully celebrating.
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The 47-year-old man died shortly after 3pm on Tuesday.
Specialist explosive detection dogs searched through the town examining letter boxes and phone booths for signs of potential devices.
Temperatures reached 29.1C in St James's Park, central London.