A Queensland woman who murdered her elderly grandfather in Adelaide has been sentenced to life in prison with a non-parole period of 21 years, with the judge describing the crime as brutal and dispassionately planned.

Key points: Brittney Dwyer claimed to be motivated by a horror television series about murder

Brittney Dwyer claimed to be motivated by a horror television series about murder Her girlfriend Bernadette Burns encouraged Dwyer to follow through with the plan

Her girlfriend Bernadette Burns encouraged Dwyer to follow through with the plan Burns was sentenced to life imprisonment for a statutory murder charge

Brittney Jade Dwyer was 19 years old when she stabbed her 81-year-old grandfather Robert Whitwell to death at his Craigmore home in Adelaide's north in August last year.

The South Australian Supreme Court heard Dwyer, now 20, claimed to be motivated by a horror television series about murder and gruesome crimes, and also wanted to steal her grandfather's life savings of more than $100,000, which he kept in cash.

Dwyer's girlfriend, Bernadette Burns, now aged 22, travelled to Adelaide with her and waited outside in the car putting her makeup on.

She encouraged Dwyer to follow through with the plan through a series of text messages, including one that read "just harden up and do it".

The court heard Burns didn't intend for Mr Whitwell to be killed or harmed, but would have contemplated that violence was a possibility.

Burns was also sentenced to life imprisonment for a statutory murder charge, with a non-parole period of 13-and-a-half years.

Family shattered by Dwyer's responsibility in murder

Justice Kevin Nicholson said the victim impact statements given by Mr Whitwell's family members were "extremely moving".

Robert Whitwell was found dead in his Craigmore home in Adelaide's north. ( Supplied: SA Police )

He said the very same people who had been badly damaged by the sudden act of violence and the death of Mr Whitwell were "simply shattered" by the fact Dwyer was responsible.

"It is not possible to adequately summarise the distress and profound anguish," Justice Nicholson said.

The court heard that while driving to Adelaide from Queensland with her girlfriend, Dwyer had second thoughts about her plan to kill and rob her grandfather but that she didn't back out.

"You failed to see or accept the enormity and abhorrent wrongfulness of what you were planning," Justice Nicholson said.

The court heard Dwyer went into her grandfather's house and sat with him looking at childhood photos, before stabbing him four times.

She then helped him apply a bandaid to one of the stab wounds in his neck, before busying herself by washing the dishes as he lay dying on a kitchen chair.

"The brutal facts concerning the murder of your grandfather are relatively straightforward, however why it came about can only be seen as very complex," Justice Nicholson said.

He said there were many factors, including Dwyer's personality disorder and drug addiction that contributed to her callous crime.

"Your unnatural interest in violence and seeing people die and greed for money are contributing factors.

"One remains at a loss to explain how a young woman of 19, notwithstanding your difficult background, although I have come across much worse, could do what you did.

"This murder was brutal, callous, cold-blooded and dispassionately planned."

Bernadette Burns was sentenced to 13 years and six months for her involvement of the murder. ( ABC News )

Justice Nicholson said he wasn't confident Dwyer had the "emotional makeup or capacity to be truly contrite".

He said a non-parole period of 29 years was an appropriate starting point.

But after allowing a discount for Dwyer's guilty plea and cooperation with police and taking into account her young age, he reduced that to 20 years and six months and added six months for the aggravated serious criminal trespass.

Dwyer did not show any emotion as she was sentenced and as the judge detailed her crime.

Justice Nicholson said he believed she posed a danger to the public and her eventual release on parole wouldn't be guaranteed, unless she could show significant progress of rehabilitation.

He said although Burns didn't have a murderous intent, she did have a serious criminal intent and knew it was a possibility violence would be used against an 81-year-old man.

He started with a non-parole period of 20 years for her, but after allowing discounts for her guilty pleas and circumstances he reduced that to 13-and-a-half years.

Robert Whitwell 'truly a real gentleman'

Outside court Mr Whitwell's brother, Geoffrey Whitwell, spoke to the media surrounded by family members, including Tonya Dwyer, who is Brittney Dwyer's mother and Robert Whitwell's daughter.

Geoffrey Whitwell described his brother as a true gentleman. ( ABC News: Candice Prosser )

"It has been a difficult time," he said.

"We support one another, we're there for one another as a family, and we just have to know now and be glad that it's all over and we can get closure and let the healing begin and just move on."

Mr Whitwell said his brother was a gentle, kind man.

"You couldn't get a better brother, he was truly a real gentleman — I don't think he ever had an argument with anybody ever in his life, he always helped everybody whenever he could and he always tried to do the right thing," he said.

"You never forget, do you? It's a test on everybody in the family and you've just got to pick yourself up, dust yourself off and do the best you can and get on with life."

Mr Whitwell said he felt the sentence was fair.

"I think the presiding judge did an excellent job ... I believe it's a fair sentence for the crime that's been committed."