Jonathan Watt and Verity Carter's father was the first man to be convicted of sexual abuse linked to the Children of God cult in Scotland.
The cult started in the US in the 1960s but Jonathan and Verity were brought up under its strict teachings in "communes" around Scotland in the 80s and early 90s.
The religious sect, which was also known as The Family, has faced allegations of widespread sexual and physical abuse of children, including sexual violence, incest, and brainwashing.
Last year, Verity and Jonathan's father Alexander Watt was convicted of four charges of sexually abusing his daughter and another child in Renfrewshire and on the east coast of Scotland when they were part of the cult.
Following his conviction, Verity, who is now 39, began to speak out about her ordeal but her younger brother has never publicly discussed his upbringing before.
Jonathan told BBC Scotland's The Nine he was not sexually abused but was the subject of "daily beatings" while growing up in the cult.
He says: "The only way you can sustain a cult is you've got to have total control. You're living in a world inside a world. You've got no contact with the outside world.
"The only way to stop contact is to kill curiosity, and the only way to do that is basically to drill into you from when you were born that you're useless, you're worthless, you're ugly. Unfortunately with a cult like that, where you are outside of the law, it's a safe haven for some abusive, violent adults."
The Children of God began in the United States in the late 1960s and built on the 'free love' philosophy of counter-culture America.
Its founder, David Berg, told members that God was love and love was sex, so there should be no limits, regardless of age or relationship.
By the 1970s Berg's cult claimed to have 10,000 full-time members in 130 communities around the world.
Hollywood stars Rose McGowan and Joaquin Phoenix were born into it.
In Scotland, it operated at sites in Renfrewshire, Lanarkshire, Ayrshire and Edinburgh.
Verity says she was abused from the age of four by her father and other cult members and she hopes that by speaking out it will encourage others to come forward to expose the actions of The Family.
During their upbringing, the siblings say they were frequently moved to different properties where "communes" had been set up.
These were often in parts of the Scottish countryside with nothing for miles around and the children were deliberately never told of their exact location.
Verity says the adults in the communes would have "cult names" - often Biblical and frequently unrelated to their true identity - which has presented issues in trying to gather evidence of their abuse.
While staying in the communes, the pair say there was some contact with the "systemites" in the outside world - especially social workers - but they were well-prepared for them.
"They were very well scripted [for us]," Verity says. "They even dressed us in nice clothes - I remember I used to wear a dress with ribbons when the school inspector was coming."
Despite being younger than his sister, it was Jonathan who was first to leave the cult.
Verity was only told that her brother had been "excommunicated for incurable demon possession".
However, Jonathan says he was ejected from the group after having a "total mental breakdown" at the age of 14.
"They tried to pray the demons out of me and it didn't work," he says. "Ultimately they had to excommunicate me - but they had to force me out."
Despite being free from the cult, Jonathan struggled to cope after he left.
He says: "I basically continued to follow The Family rules inside my own head as I couldn't accept the outside world."
Eventually, it led to him becoming homeless at the age of 16.
"I was on the streets three weeks before my 16th birthday, with a change of clothes, a bag and nowhere to sleep that night," he says. "Although it was terrifying, in a way it was what I needed."
After years of abuse, Verity also wanted to leave.
"When I hit 15, I had already tried to commit suicide on three occasions and had attempted to run away twice," she says. "I had reached the point where it didn't matter anymore because, however bad the outside world might be, what difference could it possibly make?"
She says she decided to get herself thrown out by gathering items which were forbidden by The Family such as cigarettes, make-up, jewellery and chewing gum. After being confronted by adults about her stash, Verity was deliberately unrepentant - and started laughing when they tried to perform an "exorcism" on her.
"One of the adults in the home, his name was Paul, came up to punish me and he had his belt off and he wanted me to drop my trousers," she says. "Then he started chasing me around the room, and as I was running away, I grabbed the belt off him and started hitting him back.
"I'd never done anything like this, I'd never stuck up for myself in this way. I'd said 'no' before, but I never really felt I had the right to do it. I'd never fought for myself."
When she left The Family in the early 1990s, Verity struggled with alcohol and drugs in an attempt to block out her childhood experiences.
She says: "I changed my lifestyle when I had kids but, to be honest, it is probably sheer luck that I survived those first few years out of the cult."
The pair hope that by raising awareness of what happened to them, it might encourage other survivors to contact the police and other authorities with their stories.
Verity says: "So many of them (survivors) don't feel like there's any point in speaking up about it.
"But I know for myself, when I finally did find my voice, it did give me a bit of closure."
The remnants of the group continue to have a presence online, now known as The Family International.
A spokeswoman said: "Although the Family International has apologised on a number of occasions to former members for any hurt, real or perceived, that they may have suffered during their time in our membership, we do not give credence to tales of institutionalised abuse."