WA man 'lashed out' and murdered sister

John Michael MacKinnon fatally stabbed his sister during a drug and alcohol-fuelled argument about money, then went after his nephew with the same bloodied knife, telling the teenager his mum was dead and he was next.

MacKinnon pleaded guilty to murder and causing grievous bodily harm, and was sentenced on Friday in the WA Supreme Court to life in prison with a minimum of 18 years to be served before he can be eligible for parole.

The 43-year-old had moved in with Angela MacKinnon and her 19-year-old son Jesse Henwood at their Gidgegannup home, northeast of Perth, about 18 months before the attack in March last year.

He had consumed up to 14 beers and cannabis during the day, while his sister and her son had also been drinking, the court heard.

During an argument about money that Ms MacKinnon claimed her brother owed her, she slapped him and he pushed her onto the couch.

Mr Henwood supported his mother and told MacKinnon to leave before he went upstairs to his bedroom.

Instead, MacKinnon grabbed a hunting knife and stabbed his sister several times, the court heard.

Mr Henwood heard his mother call out, then saw his uncle covered in bloodstains and coming for him with the same weapon.

He ran back to his bedroom, but MacKinnon pushed his way in and told him his mum was dead and he was next, then repeatedly slashed him.

As Mr Henwood begged his uncle to stop, MacKinnon told him he was going to die, the court was told.

After the double-stabbing, MacKinnon threw the knife away while driving towards his ex-partner's house.

Defence counsel Belinda Lonsdale said her client had significant gaps in his memory about what happened.

She said he "just lashed out and mentally lost it" but never intended to kill Ms MacKinnon.

"He loved his sister. He's still coming to terms with that - that he caused her death," Ms Lonsdale said.

"It haunts him every day."

Justice John Chaney accepted MacKinnon never intended to kill his sister and was remorseful, but said the crime had been devastating to the family.

He referred to victim impact statements that described Ms MacKinnon as a loving mother and grandmother.

Justice Chaney said the ordeal must have been terrifying for the victims and MacKinnon attacked his defenceless sister when he should have walked away from the confrontation.

Family sitting in the public gallery wept as the sentence was handed down.

Outside court, Ms MacKinnon's sister Christine McFarland said the family was satisfied with the sentence and described Ms MacKinnon as a beautiful person.