The aunt of 13 malnourished siblings who were found captive in their parents' home says they were not allowed to do "normal things kids do".

Teresa Robinette, the sister of Louise Turpin, said: "They weren't allowed to watch TV. They weren't allowed to have friends over... the normal things that kids do."

She also told NBC's Today programme she would say the children looked too skinny, but said her sister would "laugh it off" and say they take after their "lanky" father.

Louise Turpin (49) and David Allen Turpin (57) allegedly held their 13 children captive at their home in California.

Police were alerted when their 17-year-old daughter managed to escape through a window and call police from a deactivated mobile phone.

"I always made comments to Louise when I did talk to her, about, gosh, they are so skinny... she would laugh it off and say 'David's so tall and lanky, they are going to be like him'," Ms Robinette said.

The parents turned their home into a private school called Sandcastle Day School, where the father is listed as principal and its six pupils are the couple's younger children, police said.

No state agency regulates or oversees private schools in California, and they are not licensed by the state education department.

David Allen and Louise Anna Turpin are seen in mugshots released by police | Image: Riverside County Sheriff's Department

Another aunt, Elizabeth Jane Flores, told ABC News' Good Morning America programme that she was shocked by her sister's arrest.

She said she tried for years to get in touch with her but was shut out.

"I want to reach out to the kids, I want them to know that for years we begged to (see them on) Skype, we begged to see them, the whole family," she said.

Ms Flores said she and her sister did not have a relationship for two decades beyond the odd phone call.

The Elvis impersonator who helped the couple renew their vows three times has also spoken out about the revelation.

Kent Ripley told the Associated Press the story is "very disturbing".

"They all looked young and thin but I figured it was just their lifestyle. Maybe the activities they did, maybe because of their religious beliefs. I didn't get that in depth with them but I knew they were a fun family," he said.

"Shackled to their beds"

At a news conference on Tuesday, police said the investigation was ongoing and was still in its early stages.

Gregory Fellows, Riverside County Sheriff's Office police chief, said the mother seemed "perplexed" when police arrived at her home.

He said it was not clear how long the family had been held in such conditions but it appeared to have been for a "prolonged period".

Three of the siblings were found chained to furniture, he said. Six of the children are minors, while the other seven are over 18.

The children were found at their parents' home in Perris, which is about 70 miles east of Los Angeles.

Police said the children were found "shackled to their beds with chains and padlocks in dark and foul-smelling surroundings".

Their parents have been charged with nine counts of torture and 10 counts of child endangerment, the Riverside County Sheriff's Office said.

They are being held in a detention centre on $9m (€7.3m) bail and are due to appear in court on Thursday.