The sister of a woman murdered by her estranged husband says her hopes for meaningful change have been dashed after learning of attempts by the family of killer Mark Bombara to get help from WA Police.

The details of Ariel Bombara's pleas for help to stop her father hurting her and her mother have reverberated across the country, as people try to come to terms with his brutal double murder of two women in Perth on Friday.

But perhaps no one has felt the despair more acutely than the sister of murder victim Lynn Cannon, who was stabbed to death by her estranged husband Paul in Perth in December 2022.

"It's like it's being repeated again," Jacqueline Darley told ABC Radio Perth.

"Ariel had asked for help in time to save lives … it was the same as our situation, we had asked for help in time to save our sister Lynn's life."

Ms Bombara said she warned police three times her father had guns and was a threat to her and her mother, but was repeatedly ignored.

Bombara went looking for his former partner at her friend Jennifer Petelczyc's house in Floreat on Friday afternoon and when he couldn't find her, shot Ms Petelcyzc and her teenage daughter Gretyl, who was only 18, before shooting himself.

"Listening to Ariel's statement last night and my heart just went out to that poor young woman," Ms Darley said.

Striking similarities 

During her killer's sentencing hearing, the Supreme Court was told Paul Cannon had a "festering" possessiveness and selfishness and hadn't come to terms with the end of their relationship.

Cannon had threatened to use swords, though he ultimately took Ms Cannon's life, at 51, with two kitchen knives at her home in Landsdale.

Mark Bombara used a gun.

The end result was the same – women lost their lives at the hands of men.

In Ms Cannon's case, her family also sought help from WA police.

But despite warning police that morning Paul was dangerous, and multiple calls from neighbouring residents to police after he arrived at Ms Cannon's home, it took officers more than an hour to attend after the priority of the job was downgraded.

Ms Darley and her family have made eloquent, impassioned pleas for police to change the way they respond to domestic violence but the Bombara case has dashed her hopes.

"You keep being told these things are being implemented and there's so many other things in the pipeline, but how many lives have been lost since we lost our sister?" she asked.

DV history not an indicator

Paul Cannon, like Bombara, didn't have a record for family violence.

According to the WA Ombudsman, who reviews family violence deaths, fatalities are more likely when there is previous reported domestic violence.

But in 39 per cent of cases of intimate partner homicides recorded between 2012 and 2022, that wasn't the case.

Ms Darley said lessons must be learned and police need to be able to intervene earlier.

"I think it's important to [point] out that the parasite that took my sister's life, it was his first offence," she said.

"Lessons need to be learned from this, there's not always the history."