Natalie Sands was visibly emotional as she read a victim impact statement in Richard George Sands' hearing in the NSW Supreme Court on Friday.

Justice Robert Hulme found Sands physically committed the two murders and two attempted murders when he poured petrol over the Hillvue home and sparked a fire with a cigarette lighter in the early hours of October 17, 2019.

But, he found Sands was not criminally responsible due to mental health and cognitive conditions which meant he didn't know what he was doing was wrong.

Sands has been behind bars since he handed himself into police in the hours after the deadly fire, and will remain in custody.

Ms Sands and another child, who cannot be identified, managed to escape the burning home through a window.

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The court heard she suffered burns and spent time in an intensive care unit after petrol was poured over her and the house set alight with them inside.

"The answer I search for more than any other is why," she said.

"What did we do to deserve the horror you inflicted?

"I can't escape these scars on my body that I am now wrapped in."

Justice Hulme thanked her for her courage.

"No words can express what I have heard and read about the loss and the grief ... it's just unbelievable," he said.

Judgement was delivered after the court heard the Crown and defence "bundles" had already been handed up prior to the hearing, including expert medical reports, photos of the home, a sketch of the floorplan and two further victim statements.

The court heard Sands' health had deteriorated after his retirement and he was suffering mental and cognitive conditions at the time.

A set of agreed facts said emergency services responded to calls for help before dawn after flames were seen coming from the Hillvue home.

Witnesses and firefighters tried to gain entry but the intensity of the fire stopped them.

A young boy was killed, and the court heard the older woman died from blunt force head injuries inflicted before the blaze broke out.

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