The couple charged in California with shackling and starving 12 of their 13 children grew tired of their strict religious lifestyle -- so they started engaging in seedier sex-fueled hijinks, the mother’s sister revealed on Monday.

David and Louise Turpin began “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,” Louise’s sister Teresa Robinette said on “Megyn Kelly Today.”

For example, the sister said Louise, 49, had sex in a hotel with a stranger she met online — with the blessing of her husband, 57 — before re-enacting the affair in the same room exactly a year later, The New York Post reported.

Louise told her sister in a phone call around 2009 that the couple “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 claimed.

“All I know is that it did happen,” Robinette continued, according to The Post. “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.”

The Turpins have pleaded not guilty in California to multiple counts of torture, child abuse and false imprisonment, after 13 children, ages 2 to 29, were found living in deplorable conditions in the Riverside County home.

David and Louise Turpin were charged Thursday and face up to life in prison. Each was ordered held in lieu of $12 million bail.

The 12 oldest children, as Fox News has reported, showed signs of severe malnutrition, including stunted growth.

The children were being homeschooled but lacked even basic knowledge such as what a policeman was, investigators said.

Prosecutors said they didn’t know what motivated the Turpins to torture the children in such squalid conditions. The parents were quoted as saying that “God called on them” to have such a large family.

Prosecutors have said that over the years the children were mistreated, beaten and taunted with food and toys they could look at but not have.

The Associated Press contributed to this report.