The two young girls - Starlet, 15, and Angel, 11 - grew up thinking that Miss Dugard, 29, was also a daughter of their father Philip Garrido, 58, and that there was just a large age gap.

They are believed to have thought that Garrido's wife Nancy, 54, was their mother.

"They thought Jaycee was their sister," Miss Dugard's stepfather Carl Probyn told People magazine. "People have to realise this will take years of therapy."

On their rare trips outside Garrido's back yard in Antioch, California the girls repeatedly talked about Miss Dugard as a sibling.

Allison Jacobs, the police officer who raised the alarm about Garrido last week after seeing him acting strangely at the University of California, Berkeley, said the girls talked to her about their older "sister".

Angel told the officer their sister was 28 and Starlet then corrected her, saying she was 29, the officer said.

One of Garrido's neighbours, Haydee Perry, also described how Angel talked about an elder sister.

Angel came round to Miss Perry's house with her father when he was helping to change a car battery. She told Miss Perry the sister was 28.

Ernie Allen, president of the National Center for Missing and Exploited Children, said: "It's going to take patience and time to help these children."

Steve Contreras, who runs a car repair business and bought business cards from Garrido' printing business, said Angel seemed much younger than her age.

Garrido brought Angel round to Mr Contreras's shop in Antioch and she spent 30 minutes there playing with an imaginary friend.

Mr Contreras told the Telegraph: "She looked a lot smaller than her age and acted a lot younger as well, more like a five-year-old. She was talking to an imaginary friend and speaking both sides.

"He used to come in about every three months and he was really strange. He said the world was coming to an end. I thought he was on crank (crystal meth).

"Once there was a mistake on my business cards so I rang up and they were redone that day.

"Nobody does business cards that quick and I used to joke he must have some slaves in the back yard doing them."