Raelene Polymiadis, 65, is on trial in the South Australian Supreme Court charged with murdering Brenda and Lynton Anderson, who were both aged 94 when they died in March 2022 and May 2023 respectively.
In her opening address on Tuesday, defence counsel Marie Shaw KC said her client had denied administering insulin to either of her parents, and two elements in the case - identity and causation - would be disputed.
Mrs Anderson died at the Flinders Medical Centre, while Mr Anderson died after he was found unconscious on his kitchen floor.
The cause of death in both cases was insulin toxicity and the prosecution alleges that Polymiadis, a type 1 diabetic who uses insulin, was the last family member to see each parent before they became unwell.
Ms Shaw told the jury they would need to consider whether Mr Anderson's deteriorating physical condition and whether he had lost his will to live may have contributed to his death.
"He had not only talked about suicide but had stated to a support worker that he knew how he could do it," Ms Shaw said.
"The issue as to identity is whether the prosecution can exclude the reasonable possibility that Mr Anderson administered the insulin to himself."
Mrs Anderson had been admitted to hospital in February 2022 with a fractured vertebrae from a fall, complicated by pneumonia and a fractured hip from another fall.
She had previous hospital admissions for a heart scare, infected burns, a staph infection, a stroke and many other medical conditions, Ms Shaw said.
On March 16, 2022 after treatment for hypoglycemia failed to stabilise her condition, Mrs Anderson told her doctor that she "wanted to be let go", and she entered end of life care.
A key issue would be whether the prosecution could exclude as a reasonable possibility that the substantial cause of Mrs Anderson's decision was all of the medical conditions from which she was afflicted, and pain from her fracture healing, Ms Shaw said.
The prosecution would also need proof of causation.
"Was death caused by my client?" Ms Shaw said.
The defence would challenge the prosecution's reliance on blood samples and expert opinions, questioning their accuracy and potential contamination.
The first witness to give evidence in the trial was Polymiadis' brother, Graham Anderson, who said he was the "culprit" who wanted his father to move into residential care.
The Andersons' four children had been helping their father to remain in his own home after his wife's death, but he had become increasingly frail.
"My three sisters accepted the situation, because they knew that's what Dad wanted," Mr Anderson said.
"But I took the initiative to say: 'Look, Dad, you've got to start thinking about a nursing home, because we can't keep doing this nightly'."
The last time he saw his father alive, on April 25, 2023, they had argued about the issue, he said.
"He said, 'When you get old, I hope your sons put you in a nursing home', to which I replied: 'They wouldn't have to, Dad, I would put myself there long before it got like this," Mr Anderson said.
"So we were pretty angry at each other, and it wasn't resolved."
Lynton Anderson was found unconscious on his kitchen floor on April 30, two hours after Polymiadis left the house, prosecutor Michael Foundas alleged.
He was treated for hypoglycemia, but died on May 1.
The trial continues.
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