Man can't explain killing sister in law

Jian Guo Li has never denied stabbing his former sister-in-law to death, but nor can he explain why he did it.

The 49-year-old plasterer had no ill will towards his ex-wife's sister, mother of four Dong Mei Sun, and had spent the evening she died drinking and celebrating with her.

Ms Sun, 55, and Li, 49, were staying with their friend in common, Lurissa Sun in the Melbourne suburb of Narre Warren last July so the family would be together for the 18th birthday of Jack Li - Lurissa and Li's son.

Ms Sun's ex-husband Ziming Yi was also staying at the house, and the cramped sleeping arrangements meant Li did not get much sleep in the days leading up to the stabbing.

The afternoon of Sunday, July 6 the whole family went to a restaurant in Springvale to mark Jack Li's birthday, and drank heavily.

After returning home, Li and Ms Sun, who were already very drunk, continued to drink.

Around 10pm Mr Yi made some food in the kitchen where he was soon joined by Li and Ms Sun.

In the Victorian Supreme Court on Friday, Justice Cameron Macaulay said what happened next is mysteriously devoid of any explanatory context or detail.

While sitting at the table, Li picked up a 30cm serrated kitchen knife and plunged it into Ms Sun's chest, piercing her sternum and penetrating 14cm into her torso.

Mr Yi was at the table but he had his head down and did not see it happen.

After Li left the room, Ms Sun said to her ex-husband: "Jian Guo stabbed me".

Mr Yi said that was impossible.

She then stood and walked into the lounge room where she collapsed and later died.

Li went to a service station to buy some cigarettes.

When he returned, he found the house had been cordoned off by police, he raised his hands and said: "I did it okay, I did it".

But in his police interview he said nothing about the incident.

In sentencing Li to six years and nine months in prison on Friday, Justice Macaulay said Li was still unable to give any reason for it.

"The incident presents as a bizarre, random, spontaneous act without any evidence of explanatory context or build up," Justice Macaulay said.

Li must serve a minimum of four years and three months before being eligible for parole.