A groom-to-be "looked like a man possessed" before stabbing his brother, holding him a headlock and punching him continuously, a court has been told.
Bernard John Robbins is charged with murdering Gregory Robbins, 61, at a home in Buderim on the Sunshine Coast in 2019.
Three days later 59-year-old Bernard Robbins was due to get married.
His brother had arrived in Queensland from Perth on June 12 to attend the wedding.
That same day - before unpacking his travel bag - an argument broke out between the men who were drinking on the patio of the unit pensioner Bernard Robbins shared with the woman he was due to marry.
The men argued about the relationship between Gregory Robbins and his daughter, with the older brother, saying, "that's my business ... leave my daughter alone", his partner Karen Vanden Driesen told the Brisbane Supreme Court on Wednesday.
During the altercation it looked like the brothers were "going to go for each other", she said.
They calmed down later, but things flared again, escalating when bride-to-be Sally Doring suggested the wedding may not go ahead because of Bernard Robbins' behaviour.
"That's when his mood really changed and he looked like a man possessed at that point," Ms Vanden Driesen told the court.
Bernard Robbins told his brother several times to leave, later saying the couple's bags were on the driveway.
Gregory Robbins and Ms Vanden Driesen were walking out the garage with Bernard "trying to have his last words" when a scuffle broke out about 10pm.
"They were rolling around the floor, punching," Ms Vanden Driesen said sobbing on the witness stand.
She didn't remember seeing knives being used, but picked two knives off the floor, putting them in the kitchen, thinking it would help stop someone being stabbed.
Returning to the garage she saw Bernard Robbins holding his brother in a head-lock, punching him continuously and noticed Gregory Robbins bleeding.
"I was screaming, 'You're killing him, you're killing him'", she added.
"Bernard's screaming in my ear, 'Shut the f*** up, shut the f*** up'."
Gregory Robbins was stabbed in a "crime of passion", dying in hospital in the early hours of the following morning, prosecutor Christopher Cook earlier told the jury.
He had four stab wounds, eight to 12 centimetres deep, close to his heart, and was also stabbed twice in the back.
Police later estimated Bernard Robbins had a blood alcohol concentration (BAC) of 0.182 at the time of the stabbing, while his brother's BAC level was 0.217.
The trial, set down for five days, continues before Chief Justice Catherine Holmes.
