A Perth psychologist who murdered his neighbour in a planned brutal attack before killing her brother has been jailed for life with a minimum of 34 years.

Luke David Fawcett, 29, repeatedly stabbed Maree Collins, 66, with a bread knife at the entrance to her "immaculate" Victoria Park apartment on Easter Sunday in 2020.

When the widow's siblings couldn't contact her, her brother Wayne Johnson, 62, went to her home and was also attacked.

Fawcett has never revealed a motive for the killings.

He spent at least a week plotting his neighbour's murder, purchasing a taser, tarpaulin and rope, and digging a shallow bush grave 200 kilometres from Perth.

On his phone, police also found a bizarre self-shot video of him "rehearsing" an attack weeks before the murders.

Wearing a black latex suit and wielding a knife and taser, Fawcett was seen threatening to kill an "imaginary person".

After butchering Ms Collins, the psychologist made himself at home in her apartment, drinking beers and smoothies in the spare room while her body was still on the floor.

He also tried to access her bank account numerous times and sent flirty text messages to a woman about his new "penthouse apartment".

After Mr Johnson's murder, Fawcett sent the same woman a photo of himself in hospital, along with the message "sexiest patient ever".

Fawett had told medical staff he'd hurt himself falling down stairs, when really it was Mr Johnson who inflicted the injuries while fighting for his life.

With a mountain of evidence piled up against him at his trial, Fawcett tried to explain it all away through a bizarre web of lies.

But a jury saw straight through it.

In sentencing, Justice Jennifer Smith said the attacks were "sustained, brutal and merciless" and "must have been utterly terrifying".

The victims' sister Helen Batina, who discovered their bodies, told reporters outside court the sentence won't bring back her siblings.

"We've been living our worst nightmare for 22 months and it will continue forevermore, as we still remain without answers or a reason why," she said.

Fawcett's grandparents, who were also at the sentencing, revealed they are still supporting him.

"I've known him since the night he was born and he was a lovely child but he became more and more troubled as he got older," grandmother Heather Rogers said.

"We were out of touch for two years prior to the murders ... we were all terribly, terribly shocked."

Luke Fawcett will be 61 by the time he is eligible for parole.

His minimum 34-year sentence is one of the longest prison terms ever handed out in WA.