The twisted Turpin family secret that led to one of the worst cases of child abuse the world has ever seen is set to be exposed.

The depraved world of the notorious Turpin family is set to be exposed in a new documentary on one of the worst cases of child abuse the world has ever seen.

It comes as David and Louise Turpin made a brief appearance in a California court on Saturday morning for an arraignment into the case where they are accused of mistreating 12 of their 13 children.

Louise's family were interviewed by the show including sister Teresa Robinette, who is set to spill on the trauma experienced by the sisters when they were young.

"This is a house of horrors, that's where it all began," a teary Teresa says as she is filmed approaching the steps of the home where the sisters grew up.

"She took exactly what we went through and intensified it by like 100 for those kids."

However Louise's brother Billy Lambert tells the show he had no idea all wasn't well in the Turpin home.

"We thought that she had the perfect life, she had the perfect husband," he said.

Teresa previously revealed in an interview with The Sun that their mother Phyllis Robinette let a rich paedophile sexually abuse them as children in exchange for cash.

"Our mum should have been protecting us — but she sold us to a wealthy paedophile," Ms Robinette said.

"He would slip money into my hand as he molested me. I can still feel his breath on my neck as he whispered 'be quiet'.

"We begged her not to take us to him but she would simply say: 'I have to clothe and feed you'. Louise was abused the worst. He destroyed my self-worth as a child and I know he destroyed hers too."

COURT APPEARANCE

David and Louise made a brief appearance at Riverside County Superior Court on Saturday. In a sign they are trying to move on with their lives, the victims asked that their birth certificates, IDs and a camera that was removed from the home be given back to them, ABC 7 reported.

The documents had been taken as evidence in the torture case, and the judge agreed they are no longer needed and will be released back to the children.

It comes more than a month after a judge ruled that there was enough evidence for the duo to stand trial on 88 charges including torture, child abuse and false imprisonment. David also faces one charge of lewd acts on a child.

However, one charge was dropped against David in last month's hearing, when the judge ruled the youngest of the children, aged 2, did not appear severely malnourished.

The Turpins are suspected of having treated their children so badly that they now suffering long-term disabilities including stunted growth, after being found emaciated and barely educated.

One of the girls plotted her escape for two years before breaking free of a window, and called authorities to the home.

"I live in a family of 15 people and my parents are abusive," the girl told the dispatcher in a chilling 911 call that was played at a court hearing in June. "They abuse us and my two little sisters are chained up."

The children were discovered filthy and shackled inside the Perris, California house, in dark and foul-smelling conditions.

They were allegedly denied food and toys, and the oldest of the children — aged between two and 29 — weighed just 37 kilograms. They were allowed just one shower a year.

Doctors who examined the children found signs of severe malnutrition.

David and Louise had previously pleaded not guilty to the charges and are being held on $17 million bail each. If convicted, both face life in jail.

'OBSESSED WITH WITCHCRAFT'

A new book by another one of Louise's sister, Elizabeth Flores, detailed how the accused child abuser was obsessed with witchcraft, Satanic rituals and Ouija boards.

"She told me she was messing around with witchcraft. It just really freaked me out," Ms Flores told the New York Daily News in an interview about the book Sister of Secrets.

Ms Flores said her sister was obsessed with the "dark arts" and had even tried to persuade her to join a snake handling festival.

"Louise was attracted to that, like women dancing with rattle snakes around their necks."

Now she wonders if the obsession could have played a part in the child abuse nightmare.

Ms Flores, who hasn't had a "sister relationship" with Louise for about 20 years, also said she noticed her sister changed when she met David, when she was just 15 years old.

She has also previously claimed that she, Louise and their cousins were sexually abused as children by a close family member, with talk of the molestation "swept under the rug".