It’s a family affair.

A jilted Welsh husband claims he caught his wife having sex with his father on his child’s baby monitor.

Declan Fuller, 22, was married to Stephanie, 22, and they share 2-year-old daughter Willow.

Last September, the couple let Declan’s dad, Darren, 44, move into their UK home, where the alleged affair took place.

“It’s sick — how could my father do that to me?” a distraught Declan wondered during a Monday interview with The Sun. “It’s not normal.”

Declan claims he was at work on the day of his wife’s alleged infidelity and decided to check Willow’s baby monitor on his smartphone.

The footage showed Stephanie slinking into a bedroom with her father-in-law, seemingly to indulge in an afternoon romp.

Declan says the shocking act of betrayal almost caused him to fly into a homicidal rage, as he admitted: “If I didn’t have a sensible head on my shoulders he [dad Darren] would be six feet under now.”

When confronted with incriminating evidence, Stephanie and Darren denied they were doing anything untoward.

Instead, the pair claimed they were simply in the bedroom to watch “The Simpsons” together.

Declan was left unsatisfied by that explanation and promptly split from his spouse — only for her to embark on a romance with her father-in-law.

“I did not cheat on Declan,” Stephanie insisted when contacted for comment by The Sun. “Darren and I got together a week or so after Declan left me.”

In a stunning twist, Stephanie is now pregnant with twins fathered by former father-in-law Darren.

“I feel sorry for the babies, they are coming into a very mixed-up family,” a betrayed Declan declared, noting Willow will be a half-sister and an aunt to the soon-to-be-born twins.

Despite his heartbreak, Declan says he’s managing to move on from the family feud.

“I feel betrayed, but I’m better off without the two of them,” he insisted.