Cyrena Barned said outside court that she was pleased with the 31-year sentence received by Dean Smith, for the murder of her brother Aarron McKoy.
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.
A gunman was today jailed for a minimum 31 years for killing a teacher's son celebrating his birthday in a wine bar.
Aarron McKoy was with his twin brother Lee and it was their 22nd birthday in the early hours of New Year's Day when he was shot three times.
Dean Smith, 26, of Stoke Newington, north London, was found guilty of murder at the Old Bailey last week. He was jailed for life and told he would serve a minimum of 31 years.
– Judge Wendy Joseph
It was a true execution of a young man who was utterly defenceless and at your mercy.
Aaron had done nothing to you or indeed anyone else.