The son of wife-killer Borce Ristevski is claiming credit for his father's surprise decision to plead guilty to manslaughter.

Borce sensationally changed his plea yesterday on the eve of his five-week Melbourne trial for the murder of 47-year-old Kiwi Karen Ristevski, his wife of more than two decades.

But Anthony Rickard, Borce's son from a previous relationship, says his father's backflip had nothing to do with his conscience and everything to do with the taboo relationship Rickard claims he had with his stepmother Karen, news.com.au reports.

"I am responsible for this, I gave him an ultimatum," Rickard told news.com.au in a phone call this morning.

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"I told him; 'If you don't get up and be a man I'll go into the (witness) box and tell them exactly what went on behind closed doors'."

By "behind closed doors", Rickard says he is referring to an alleged long-running sexual relationship with his stepmother he says began when his father took him in as a troubled teenager.

The suggestion is that Borce opted out of a trial to spare his daughter Sarah Ristevski, now 23, the agony of hearing her estranged brother reveal sordid details of his alleged affair with her mother.

Rickard made similar claims of an alleged affair with Karen in 2016 when she went missing. They were not corroborated at the time.

Anthony Rickard says his stepmother’s death was linked to tensions over his sexual relationship with her. Photo / news.com.au
Anthony Rickard says his stepmother’s death was linked to tensions over his sexual relationship with her. Photo / news.com.au

Sarah has been her father's most loyal supporter, boycotting police early in the investigation in protest of their "tunnel vision" in labelling her father their prime and only suspect in her mother's murder.

Borce, 55, admitted to the killing a day before a Supreme Court jury was due to be empanelled for his murder trial, which was set down for five weeks.

It should be noted the confession followed pre-trial argument on Wednesday when the court found there was insufficient evidence to prove he intended to murder his wife.

The defence had argued that Karen's death was a spontaneous killing rather than pre-meditated.

Borce Ristevski sensationally changed his plea yesterday on the eve of his five-week trial for the murder of his wife Karen. Photo / Facebook
Borce Ristevski sensationally changed his plea yesterday on the eve of his five-week trial for the murder of his wife Karen. Photo / Facebook

"The first 14 years of my life were bad but they weren't the worst part of my life," Rickard told news.com.au.

"I thought things were going to get better when I went to live with them. I was a f***ed up kid and they were meant to 'save' me but it turns out that my mother was the good one.

"My mother actually had morals but when I went to live with [Borce and Karen Ristevski], all the morals she put in me turned to s***. I lost my self respect, I lost everything that was good.

"He knew what was going on between me and Karen but he did nothing, he didn't protect me, he was a coward.

"That's why I told him if he didn't man up now, after all these years of doing nothing while I went through hell, it would all come out in the witness box."

Rickard claims trauma stemming from a dysfunctional family life led to run-ins with the law and drug abuse. However, he remains convinced the threat of his testimony was the reason for his father's backflip.

Rickard claims he first approached his father about "the situation with Karen" in 2006.

"He told me that we'd work it out as a family, but nothing ever happened," he said.

And so it festered, he said, unaddressed, until a few weeks before Karen mysteriously vanished from the A$1 million Avondale Heights home she shared with her husband and their daughter Sarah on June 29, 2016.

Borce told police she had stormed out after an argument about money but police could find no witnesses or CCTV footage of her leaving the house that day.

For the next eight months Karen's fate was the subject of countless theories, the most bizarre of which originated from her husband's family.

Karen Ristevski with daughter Sarah Ristevski, who stood by her dad. Photo / via news.com.au
Karen Ristevski with daughter Sarah Ristevski, who stood by her dad. Photo / via news.com.au

Weeks after she vanished, Rickard went public with his claims of an affair with Karen, insisting the pair had plans to "run away together" before she inexplicably vanished.

He posted about it on Facebook and also spoke with A Current Affair.

At the time A Current Affair said they gave him a fake offer of A$50,000 to tell his story but claimed he turned it down and said he wanted A$200,000.

Rickard denied this telling the Herald Sun the programme approached him with a money offer and he did not ask for cash.

He told police he initially believed his stepmother had fled voluntarily, citing a phone conversation the pair had two weeks earlier that triggered discord within her marriage.

Rickard told cops he'd followed up the call with a text message in which he threatened to kill himself.

"When I first saw that Karen was missing, I initially felt guilt, thinking my call to her may have had something to do with it," he said at the time.

"Although I've had some serious issues with Karen, she was always there for me."

Rickard later elaborated on the phone call to news.com.au, claiming it had related to exposing an alleged sexual relationship with his stepmother.

Borce Ristevski's older brother Vasco told reporters Karen had likely fled overseas, possibly to China, on a "fake passport" after tensions between her stepson, her husband and herself reached boiling point.

"I don't think she will come back. I reckon she's gone for good," he said.

Borce, by this stage the only suspect in his wife's presumed murder, suggested she could have been kidnapped and murdered by a predator and questioned why Victoria Police were not warning women about going out alone.

Her badly decomposed remains were found wedged between two logs in bushland at Mount Macedon Regional Park in February 2017. Borce was charged with her murder in December that year.

He maintained his innocence until yesterday, when he pleaded guilty to a lesser charge of manslaughter.