Nicolette and Hardus Lotter acted with diminished responsibility when they murdered their parents, Judge Shyam Gyanda found in the Durban High Court on Monday.
"I can't envisage how two normal people from a middle class home could go about executing their parents... but for that influence," he said in sentencing them.
Last week, Gyanda convicted Hardus, Nicolette and her ex-boyfriend Mathew Naidoo of murdering Maria Magdalena "Riekie" Lotter, 52, and her husband Johannes Petrus "Johnny" Lotter, 53, in their home in Durban on July 19, 2008.
On Monday, he sentenced Nicolette to 12 years' imprisonment for each of the murders, the sentences to run concurrently. She would be eligible for parole in 10 years.
Hardus, 23, was jailed for 10 years' on each count, also to run concurrently. He would be eligible for parole in six years.
Gyanda said he had taken into account that they had spent 44 months in prison awaiting trial.
Society would be aghast if the siblings were to get away with a slap on the wrist and either correctional supervision or a suspended sentence, he said.
He said like-minded people had to be deterred from committing crimes like this or else a belief in witchcraft and the occult, such as Nicolette's, would allow people to escape liability.
Imposing two life sentences on Naidoo, 25, the judge said there were no compelling circumstances in his case to depart from the prescribed sentence of life imprisonment.
He found that Naidoo was the mastermind behind the double murder plot.
Gyanda said there was enough evidence to confirm that the Lotter siblings had been influenced by Naidoo to the extent they claimed.
He said Naidoo's assault on Nicolette had been of a sexual nature and that evidence that he forced her to drink his urine had been undisputed.
"And if she could stoop to such low levels to obey him, it follows she did exactly what he wanted just to please him," said Gyanda.
The Lotters had testified that Naidoo told them he was the third son of God and that God wanted their parents dead for their sins.
The original plot had been to taser the Lotters' parents to knock them out then inject them with airbubbles to bring about heart attacks.
When that went wrong, Naidoo had told Nicolette to stab her mother and Hardus to strangle his father.
The judge said that if analysed, the entire case indicated that Naidoo was a cunning, conniving man who aimed to harm the Lotter family.
He had stolen money from them and had sent them threatening letters and cellphone messages.
He had bought surgical gloves and the needles with which to inject the Lotter couple, and when that plan went wrong told the siblings how their parents had to be killed while he went about setting up an alibi and keeping out of harms way.
The murders were terrible and had removed two useful and productive people from society for no apparent reason, said Gyanda.
Mrs Lotter was a teacher and her husband was a chemical engineer.
Earlier, Johnny Lotter's brother, Reverend Willem Lotter of the Dutch Reformed Church in Cape Town, gave evidence in mitigation of sentence.
He said that if the siblings' own parents could have stood in court to give evidence in mitigation for them, they would have.
They had been devoted parents, had loved their children and had done their best for them.
He urged the brother and sister to take responsibility for their actions and to stop shifting the blame.
He told them that the high court of heaven had found them guilty of dishonouring their parents and of killing them.
The two became emotional when their uncle recalled their mother's last words: "I love you my children"; and their father's last words: "My son we can work this out."
He said most of the family were still grieving and many suffered from post traumatic stress disorder and had received psychological treatment.
The children's grandfathers were both in their 80s and were ill.
Their grandmother had stopped speaking after the murders and had died soon after from a stroke.
Lotter said he had shared his brother and sister-in-law's agony and concern at receiving anonymous death threats (from Naidoo).
Security cameras were installed and his brother's company was so concerned it sent a private investigator from Germany to investigate.
Lotter said he and his sister Antoinette had visited the siblings in prison on Hardus's 21st birthday, which had started their process of forgiveness and reconciliation.
He apologised to the siblings in court for not giving them water when they were thirsty, for not visiting them in prison and not providing them with respectable clothes for their court appearances.
Although he hated what they had done, he still loved them.
"I would like to say I am committed to forgiving you."
Nicolette and Hardus needed intensive pastoral and psychological counselling, he said.
Nicolette had been abused, molested and programmed to kill, which she had and she now had to get over the symptoms of post traumatic stress disorder.
"My main concern is that to this day she is still preoccupied with witchcraft. I am concerned this needs to be addressed in the strongest way."
The evidence of clinical psychologist, professor Lourens Schlebush, had helped him understand how the incident had come about, he said.
He hugged his nephew and niece before they were taken away to start serving their sentence.
National Prosecuting Authority spokesman Mthunzi Mhaga described the sentences as appropriate.
"It can never be overemphasised how important human life is," he said in a statement.
"We... accept that Naidoo had manipulated the two in committing these crimes, hence the disparity in sentences."
