Forced to have sex with my sis; Kids' home abuse claim.
The youngsters' ordeal happened when Gordon Knott suggested they play hide- and-seek, it was claimed.
And the girl, who was six at the time, alleged Knott also raped her.
At the High Court in Edinburgh, they both wept as they described the incident.
The woman, now aged 27, alleged Knott threatened that her brother would be taken away if she told anyone what had happened.
Choking back tears, she said: "I didn't want to lose him. He was everything - he was my mum and my dad."
Her brother, now 29, said they were both at Clerwood children's home in the capital between 1971 and 1976.
He claimed Knott first abused him in a bathroom in the house when he was four or five and that type of incident continued for two years.
Asked what Knott had said, he replied: "If I told anyone, I would be moved to a different home from my sister and I wouldn't see her again.
"He said not to tell anyone as it was our secret and I was his special friend."
The witness claimed the sex incident with his sister happened in stables within the grounds of the home.
Knott took off his sister's clothes before committing indecent acts on both of them.
He alleged: "She was crying a lot. He was making me lie on top of her and have sex."
He said he had kept what happened a secret for years before telling his wife.
He claimed: "I just tried to forget it. As well as that, I didn't know if my sister would be able to handle going to the police so I just kept it with me. I'll never forget what he did."
His sister, who broke down several times while giving evidence, said Knott's was "not a face I could forget".
The woman alleged he raped her and carried out other sex acts on her and when she cried he didn't stop.
Asked how many times he had abused her she replied: "I don't know. It felt it happened all the time."
Referring to the stables incident she said Knott had suggested she go there with her brother to play hide- and-seek.
"He said if I said anything, no one would believe me anyway and they would take my brother away," she added.
Earlier, another alleged victim said she was only 10 when Knott made her carry out a sex act on a frightened teenage boy.
The woman, now 33, said Knott "just sat with his arms folded and watched what I was doing to him".
She said: "My instincts as a child told me to be afraid of him and I was afraid of him."
Knott, 43, of Carnwath, Lanarkshire, Brian Maclennan, 51, and Michael Cull, 43, both of Edinburgh, deny abuse charges at three homes in the capital between 1973 and 1986.
The trial continues.