A Perth father who stabbed his son to death before watching the AFL grand final with friends has been sentenced to life in jail with a minimum of 18 years.

Ernest Fisher's two children, who he left to dispose of their brother's body, have also been jailed over the murder at the family's suburban home, but could walk free in weeks due to time already served in custody.

Key points: Ernest Fisher sentenced to life with 18-year minimum for murdering his son

Ernest Fisher sentenced to life with 18-year minimum for murdering his son Son Joshua Fisher-Turner jailed for four years for covering up the crime and burying his brother

Son Joshua Fisher-Turner jailed for four years for covering up the crime and burying his brother Daughter Hannah Fisher-Turner eligible for release for covering up the crime, after time already served

The 67-year-old Fisher fatally stabbed his 23-year-old son Matthew Fisher-Turner in October 2016 following what he claimed was a series of physical assaults.

He then went to a friend's house to watch the AFL grand final, leaving his children Joshua and Hannah Fisher-Turner to bury the body in the backyard and clean up the crime scene in Joshua's bedroom.

Matthew's body was not discovered until four weeks later after his friends, who had repeatedly quizzed Mr Fisher about his son's whereabouts, reported him missing to police.

Stabbing was 'the right thing'

Fisher admitted stabbing his son but claimed he acted in defence of himself and his children.

He claimed his son was aggressive, abusive and "terrorising" the family.

In an interview after his arrest, Fisher joked and laughed with police, describing Matthew as "a complete arsehole" and saying while it "wasn't pleasant" he felt he had done "the right thing" by stabbing his son.

The jury took just hours to convict the trio over Matthew's death. ( Facebook: RIP Matthew Fisher-Turner )

In June, after a 10-day Supreme Court trial, a jury rejected his claims and found him guilty of murder.

Fisher's barrister Sam Vandongen accepted that his client was facing a lengthy minimum jail term and that "he was facing the real prospect of dying in prison".

Mr Vandongen said tests on his brain showed Fisher was suffering "small vessel disease" which affected the functioning part of his brain.

Father forced children to make appalling choice: judge

Justice Stephen Hall described Fisher's actions as "callous, disturbing and reprehensible".

He said he did not accept the 67-year-old's argument that he had no other choice than to kill his son, saying he had "simply chosen the option that suited him best".

The murder was motivated by "hatred, frustration and anger", he said, noting Fisher had shown no remorse or regret for what he had done.

He repeatedly talked about Matthew in derogatory terms in his police interview.

Justice Hall also lashed out at Fisher for what he called his "reprehensible" decision to involve Joshua and Hannah in the crime, saying he had presented them with an "appalling dilemma" when he left them with the body.

Joshua, a 28-year-old IT student, and Hannah, a 21-year-old who was studying fashion, were convicted of being accessories after the fact.

Hannah Fisher-Turner helped clean up the crime scene and bury her brother's body. ( Supplied: Facebook )

Addressing Joshua and Hannah, Justice Hall said: "Your father effectively forced you make a choice".

"Your options were to call the police or help your father. He should never have put you in that position.

"It was an appalling abrogation of his parental responsibility."

However he said the two children did have a choice, but they made the wrong one "out of misguided loyalty to your father who failed you very badly".

Joshua sobbed as Justice Hall noted he did what he did, in part, because he had wanted to protect his sisters, saying he accepted the 28-year-old was remorseful and had good prospects for rehabilitation.

Joshua was sentenced to four years jail, but with time already served he will be eligible for release on parole in October.

Hannah received three years and eight months, but again with time served will be eligible for release by the end of the month.

Sister 'haunted' by brother's death

Their trial was told the family had fallen apart after the death in 2014 of Fisher's wife and the children's mother.

The home, in the Perth suburb of Parmelia, was described as "a pigsty".

Joshua Fisher-Turner's room where his brother Matthew was murdered by his father. ( Supplied )

Joshua and Hannah were said to be reclusive and very different from Matthew, who the court heard was an outgoing gym junkie who was seeing three women at the time of his death.

In sentencing submissions on Thursday, Hannah's lawyer, Jim Brash, said a psychological report found his client was suffering "unresolved grief from the death of her mother" as well as distress from the death of her brother.

Mr Brash also revealed his client had sent a letter to the judge expressing her remorse for what she had done.

"Being in jail, I've had a lot of time to sit and reflect … and the events of October the first 2016 fill my heart with pain and sadness … and haunt me," she wrote.

"I now acknowledge they were wrong and I am deeply sorry. My mother would also be very disappointed in me and I wish I could take it back."

Joshua's lawyer, Marco Tedeschi, said his client was also distressed by what had happened, and had run out of the room as soon as he saw his father with the knife.

"Seeing your father with a knife and not long after seeing your brother lying dead on your bedroom floor … fear would have contributed to his poor decision making that day …" he said.

Mr Tedeschi described the household as dysfunctional and said the presentence reports indicated Joshua had a lack of coping skills and "chose to hide and avoid the reality of what was happening".

Joshua Fisher-Turner was an introvert who was a very different personality to his outgoing brother. ( Facebook: Joshua Fisher-Turner )

Outside the court, Mr Tedeschi said his client was relieved the case had ended.

"This has obviously been a horrific experience for this family," he said.

"Obviously he [Joshua] was incredibly remorseful and upset by the whole thing, he's obviously been traumatised by the what happened, greatly traumatised by it."

Mr Tedeschi said he was hopeful Joshua had good prospects for the future.

"I'd be hopeful that in three months from now he can get his life together again," he said.

"I think he can get his life back on track, he's only a young man and he's got a lot going for him."