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 a year apart.
Opening the trial on Monday, prosecutor Michael Foundas said the "precious time" the couple had left together was deliberately cut short by their youngest daughter.
"Polymiadis deliberately administered fatal doses of insulin to her parents, killing them," Mr Foundas said.
"She poisoned them to death - she murdered them."
Polymiadis, a type-1 diabetic who requires insulin injections, was the last family member to see her parents alive, Mr Foundas said.
The couple were living independently in their Hackham home in Adelaide's south in 2022.
"Neither wanted to go into residential care facilities. Indeed .... they were vehemently against the idea," Mr Foundas said.
"One potential motive ... is perhaps some misplaced idea that killing her parents would spare them (from a nursing home)."
After Mrs Anderson had a fall in February 2022, she was admitted to Noarlunga hospital with a fractured vertebrae, complicated by pneumonia and another fall.
Her condition eventually improved, but the prospects of her having to move into a residential care facility on release were "starting to loom large", Mr Foundas said.
After Polymiadis visited her mother on March 12, nursing staff found Mrs Anderson "largely unresponsive".
She was treated for hypoglycemia and transferred to Flinders Medical Centre, where her condition continued to deteriorate.
On March 16, the decision was made to begin end of life care and Mrs Anderson died hours later.
The prosecution alleges Polymiadis twice administered insulin to her mother - on March 12 and between March 13-15.
"It was the insulin that killed her," Mr Foundas said.
The family was told the matter had been referred to the coroner, but there was no suggestion of a broader police investigation, he said.
"This effectively meant Mrs Polymiadis ... was operating under the belief she'd managed to murder her mother undetected," Mr Foundas said.
Sixteen years prior, Polymiadis had expressed to a previous partner that "insulin is the perfect murder weapon … because it's natural in the body and can't be detected", Mr Foundas said.
Mr Anderson continued to live at home after his wife's death, but became more frail and his four children began staying there at night on a shift basis.
Polymiadis left the house on April 30, 2023, after staying with her father for the weekend.
Two hours later, a carer found Mr Anderson unconscious on the kitchen floor, Mr Foundas said.
He was treated for hypoglycemia, but died on May 1.
A post-mortem found Mr Anderson also died as a result of insulin toxicity.
Polymiadis was the only one of her siblings to suffer from diabetes, "and importantly, neither of her parents were diabetic", Mr Foundas said.
When detectives later asked Polymiadis if she had "effectively assisted her parents to commit suicide" by supplying or administering insulin, she denied it.
"When Mrs Polymiadis was asked if she had murdered either or both of her parents, she emphatically denied that allegation," he said.
When police seized Polymiadis' phone, it showed she had Googled the sedative oxazepam on April 29, Mr Foundas said.
After Mr Anderson was admitted to hospital, toxicology results showed the sedative present in his blood.
The 12-week trial, before Justice Sandi McDonald and a jury, will hear evidence from family members, medical experts and hospital staff.
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