Debbie Breen cried as she described the horror of knowing her son murdered his father and stepmother and planned to kill more relatives, including her, but for being arrested by police.

"He was coming to kill me. This realisation haunts me every day," Mrs Breen told the NSW Supreme Court on Friday.

Corey Alan Breen, 28, pleaded guilty in June to murdering his father Paul Breen, 55 and stepmother Felicia Crawford, 45, by stabbing them in a horrific attack inside the couple's home at Killarney Vale, on the Central Coast, on March 29, 2013.

The former security guard allegedly told police he planned to kill his mother and grandparents and then travel to Sydney to kill his siblings but was forced to surrender to police when the clutch of his stolen car failed during his Good Friday crime spree.

Mrs Breen and her daughter Ashleigh were among several relatives who gave heart-wrenching victim impact statements during a sentencing hearing before Acting Justice Jane Mathews.

She said she could not fathom how her two other children "have grown into lovely people" while Corey was abusive and estranged himself from his family.

"I believe he knew how much I loved and cared for him. A mother's love is not withdrawn easily.

"[But] nothing was ever good enough for him. I have lived in fear of upsetting him, tiptoeing on egg shells.

"What if he had made it to my place [that day]?"

Ashleigh Breen said Ms Crawford was a "lovely, caring, gentle woman" who had been her stepmother since she was nine.

"Felicia was always there for me. I miss her so much."

She said she had been diagnosed with post-traumatic stress disorder.

"If I had been there [that day] … I would be dead too," she said.

Clutching her father's St George Illawarra Dragons jersey, she said she had become close to him after a difficult childhood.

"At the time of the murder he was 10 years sober. I was very proud of him. We had started to get to know each other. We even became close," she said.

She said Corey "should have been my big brother, my protector" and described the criticism her mother had endured since the murders.

"Maybe now that they know he was planning to kill her, too, people will give her a break," Ms Breen said.

Mr Breen's father Colin said Norma, his wife of 58 years, died nine months after their son was murdered.

"I believe she died from a broken heart," he said in a victim impact statement read on his behalf by a member of the Homicide Victims Support Group.

He said he gave Corey $8000 when he was in trouble with police and now wonders if a stint in jail would have prevented him from going on to commit such violent crimes.

"To think our grandson wanted to kill us when we did all we could do to be good grandparents," he said.

"I would ask Corey what the reason was but I guess no reason would ever make sense to me."

Breen's barrister Philip Young, SC, said his client had typed a two-page statement to be given to the court.

Forensic psychiatrist Olav Neilssen has assessed Breen and will be cross-examined by the Crown when the sentencing hearing continues next month.

Outside the court Paul Breen's brother Tony Breen said: "If he's not put in jail for life, God help him when he gets out."