The trial was halted after Mukhtar’s brother launched himself at the defendant, leaving jurors “extremely shaken”. Owusu had entered the witness box to give evidence in his defence.
Abas Mukhtar, 28, was fined £1,000 for contempt of court.
Judge Richard Marks KC said: “It goes without saying, having seen the incident at close quarters, I was absolutely appalled by the incident.”
Defence lawyer Charles McCombe apologised on behalf of Mukhtar for disrupting the court, jurors and his parents, and for “messing the trial up for his sister”.
He added: “I am told he was overcome by anger as to what he was witnessing.”
Marks said: “Whilst I fully understand the extent of your emotions for what happened to your sister, to behave in that way in the middle of the trial while the defendant was giving evidence is inexcusable and had potential to derail the trial.”
Details of the incident can be reported now Owusu has been found guilty of murder.
Jurors were not told that Owusu was jailed for eight years in 2010 for being the getaway driver in a botched robbery in which a Matalan shop manager was stabbed to death.