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.