Emily Thompson was just 18 and wildly infatuated with her 26-year-old half-brother when she joined him photographing and raping three young children.
First police caught Matthew Thatcher through a sting. He was apprehended after attempting to arrange a meeting with what he thought was a 14-year-old girl he'd been seducing over Facebook.
Prosecutor Suzanne Thomas told a Wales court the Facebook chats had gradually become more sexual in nature. Then he attempted to organise a tryst, said news.com.au.
The 26-year-old turned up at the rendezvous. The woman behind the messaging then called in police.
As part of their investigation, police checked Thatcher's phone.
It contained a shocking revelation: hundreds of explicit photographs revealed he was in a sexual relationship with his half-sister Emily Thompson, 18.
Worse, the pair of them were part of a trio sharing explicit photographs and text messages about their sexual exploitation of three young children.
"The images and messages showed a careless disregard for the welfare of the children," Judge Eleri Rees of the Cardiff Crown Court declared.
Emily Thompson told her lawyer that she had been "blinded with her infatuation" of her brother. She insisted she was in love with him and "did not view him as a relative".
The siblings — and another woman, Mandy Wright, 28 — have now confessed to 36 charges relating to the sexual abuse of three children aged less than 16.
The judge said in his summing-up that Thatcher had "descended into the darkest of places. He discovered that he was capable of doing the most dreadful things. He was caught up in something that he found very difficult to get out of. It was a dreadful thing that was taking place".
Thatcher has been jailed for 26 years, Thompson for 12 and Wright seven.
The court was told one of the three young victims showed signs of "chronic sexual abuse".
"Absorbing the detail and the nature of the abuse committed by these individuals has been harrowing for even the most experienced professionals involved in the investigation," detective Karen Thomas of the South Wales Police said.
"I would like to take this opportunity to publicly commend the bravery of the victims in this case — they have experienced things which nobody should have to endure."