A carer and gambling addict who murdered the elderly man she was meant to be looking after will almost certainly die in custody after being sentenced to at least 19 years in jail.

Kerry Forrest, 55, gave William Anderson, 84, a lethal dose of morphine in his food in April 2010 while he lay in a motel room in Campbelltown.

The court heard that Forrest is now in the final stages of terminal cancer.

The NSW Supreme Court heard on Thursday that Forrest had used her position of control over Mr Anderson's financial affairs to misappropriate the proceeds from the sale of his home.

She then lost a vast proportion of the money in a matter of months by pouring hundreds of thousands of dollars into poker machines at licensed clubs.

In sentencing Forrest to a maximum of 25 years with a no-parole period of 19 years, Justice Peter Hidden said she killed the grandfather to prevent the misappropriation from being exposed.

"She [then] attempted in a variety of ways to cover her tracks," Justice Hidden said.

"She tried, unsuccessfully to persuade her own GP to come to the hotel room, in the hope that that doctor would certify the death as natural causes.

"After the event, she tried unsuccessfully to arrange for the cremation of Mr Anderson without the knowledge of his family ... It was not until 14 February, 2011 ... that she was arrested and charged with murder."

Forrest pleaded not guilty to murder but was convicted in a judge-alone trial in May this year.

Forrest appeared in court from jail via audio-visual link and sat slumped over in a wheelchair as the sentence was handed down.

Justice Hidden said doctors had predicted she had only about 18 months to live, and would spend the last months of her life in the "very basic", "isolated" conditions of Long Bay forensic hospital.

"It is open to the executive government to allow an offender early release in the exercise of the prerogative of mercy," he said.

"The State Parole Authority also has the power to direct the release of a prisoner.

"Of course, it is not within my power to exercise either of those discretions. I must sentence Ms Forrest according to accepted principles, having regard to the gravity of the crime.

"The inevitable result is a sentence with a minimum custodial term extending well beyond her life expectancy."

Members of Mr Anderson's family and Forrest's family cried as the sentence was handed down before leaving the court without comment.