Accused California parents-from-hell David and Louise Turpin had their older children babysit their younger ones while they embarked on kinky sexcapades, Louise’s sister alleged Monday.

The couple’s sexual hijinks included Louise having seedy romps in a hotel with a stranger she met online — with the blessing of her husband — and then she and her hubby re-enacting the extramarital affair in the same room exactly a year later, the sister said.

Louise told her sister, Teresa Robinette, in a phone call around 2009 that “her and David had met a man online from Huntsville, Alabama, and that they were on their way there to meet him and that she was going to sleep with him and that David was okay with that,” Robinette told “Megyn Kelly Today.”

“All I know is that it did happen,” Robinette continued. “She met this man at a motel, slept with him, and what makes it even worse and even weirder is that exactly one year to the day of the anniversary that she did that, she called me and thought it was funny that David was taking her back to the exact same hotel room — the exact same bed that she slept with this man in — so that David could sleep with her in the same bed.”

Apparently weary of their button-down religious lifestyle, the Bible-thumping couple began to cut loose about a decade ago, “experimenting in different religions” and forcing their older children to “take care of the younger children so that her and David could kinda sow those wild oats that they didn’t sow when she was younger,” Robinette said.

The Turpins were arrested for torture, child abuse and false imprisonment last week after their 17-year-old daughter escaped their suburban Perris, Calif., home and alerted authorities that she and her 12 siblings were starving and being held against their will.

David — who at 24 abducted 16-year-old Louise before getting her preacher dad’s blessing to marry the then-teen — is also accused of performing at least one lewd act on a child under the age of 14, California prosecutors have said, declining to give further details.

Robinette also revealed that a close relative sexually abused her, Louise, their mother and several members of her family.

“A very, very close family member that we should have loved and trusted — he . . . sexually abused my mother and then me and Louise, [sister] Elizabeth and a few of our cousins in the family,” she said without revealing the abuser’s identity.

Meanwhile, new details about the disgusting conditions in which the family lived emerged.

Ricky Vinyard — a neighbor of the Turpins while they lived in Rio Vista, Texas, from 1999 to 2010 — told the UK’s Express that the family’s home “was waist-deep in filth” when they left.

He discovered two hungry Chihuahuas eating from a pile of soiled diapers and also noticed practically everything in the house was kept under lock and key, including the refrigerator and a toy chest.