Brenda Anderson, 94, died in hospital in March last year.
Her husband Lynton, also 94, died after being found unconscious by a carer at his home in May this year.
Police allege both died from drug overdoses from intentionally administered medication.
Last week, Detective Inspector Mark McEachern said concerns over Mrs Anderson's death were first raised during a coronial investigation, which unearthed alarming toxicology results.
A similarly concerning toxicology report was returned after Mr Anderson's death.
On Tuesday, police said a 62-year-old woman, who was a family member, had been arrested and was expected to be charged with two counts of murder.
Det Insp McEachern said detectives had no concern with the level of care Mrs Anderson received while in hospital, and had also ruled out carers being involved in Mr Anderson's death.
Police asked to speak to anyone who had contact with the couple, particularly those with knowledge of any prescription or non-prescription medication they were taking.
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