The widow of a NSW police officer stabbed to death while trying to negotiate with a violent mother and son in Sydney’s west says his murder left “a hole” in her family.

Detective Inspector Bryson Anderson was stabbed several times with a 15cm hunting knife by 21-year-old Mitchell Barbieri at a house in Oakville in December 2012.

Barbieri will spend at least 26 years behind bars after receiving a 35-year sentence for the police officer's murder. His mother Fiona received a minimum jail sentence of seven years and six months for manslaughter.

Following the sentencing, the slain officer’s wife read a short statement, saying the impact on his death had been “immense” for her and her family.

“On the sixth of December, 2012 the path of our lives was forever changed and while this outcome today is the end of the legal process – tomorrow we’ll still wake without Bryson,” Donna Anderson said.

“His murderers have left a hole in our family that will always remain.”

She also praised Inspector Anderson’s work colleagues at the Hawkesbury Local Area Command who had to continue their work after learning of his death.

“Our family would like to thank those members of the NSW Police Force who supported us and cared for us, many of whom were dealing with their own grief at the time,” she said.

"No sentence could adequately reflect the harm that has been done but we also believe the sentences imposed should be a reflection of how much our community and our police have been effected by Bryson’s death and how much we value the work the police do to keep us safe.”

The NSW Supreme Court heard the pair barricaded themselves inside their house after a violent dispute with their neighbours, in which Barbieri fired a number of arrows at their neighbours and a hired electrician.

In handing down his sentence, Justice Robert Hulme began by paying tribute to the "loyal, caring, humble, ethical, honest ... good man".

On the day their trial was due to start this year, Barbieri pleaded guilty to murder while his mother pleaded guilty to manslaughter on the basis of substantial impairment.

She sat expressionless as Justice Hulme recounted how she was once a senior executive at American Express, but by about 2008 had experienced late-onset paranoid schizophrenia.

She became increasingly paranoid she was being persecuted by police and political leaders, and her life began to fall apart.

By the time of Insp Anderson's death, her son had taken on her delusions.

Since being in prison, he was unable to understand what he had been thinking and was "heartbroken", the court heard.

But Justice Hulme said although the Barbieris regretted what had happened, he was not persuaded either was genuinely remorseful because they had not accepted responsibility.

Mitchell, he found, had also intended to kill Insp Anderson that day.

"Of all the weapons that he could have taken up, he chose the one most capable of inflicting a lethal injury."

But he said due to Mitchell's cognitive impairment, he should not be subject to the mandatory life sentence reserved for people who murdered police.

© ninemsn 2014