Debby first met her estranged brother when she was 36. Then they started having sex.

For most of her childhood, Debby Zutant lived within walking distance of a half-brother she had no idea existed. Debby had been adopted as a baby, and brought up in New York by a family who loved her.

It wasn’t until she was 36 that she decided to find her birth parents. It led her to her father, Danny, who had been in a car accident and had an intellectual disability. When she finally met him, however, he remembered her, and had been carrying a photo of her around in his wallet ever since she was born.

After breaking up with Debby’s mother, Danny had gone on to start a relationship with another woman. Together, they had a son named Joe, who was fourteen years young than his half-sister.

Speaking to Daily Mail, Debby described the first time she met Joe in 2002. “We just looked at each other and it was like instantaneous, the attraction,” she said. “It’s not even about the sex. It’s a bond unlike anything. Nobody can understand this bond it’s so strong.”

They decided to have dinner together, to get to know each other, and later, Joe visited Debby at her home. It was at this point that they started having sex.

“I freaked out. I was like, ‘What is wrong with you?!'” Debby told Daily Mail. “We were both like, ‘What have we just done?'”

At first, they kept their relationship a secret. Despite Joe moving in with Debby, and the two continuing to have sex, they didn’t tell family or friends.

“Joe wasn’t comfortable with people knowing about it,” Debby said. But over the years, she started telling close friends, who were overwhelmingly supportive.

A decade into their relationship, Joe and Debby moved to Florida together, and forged an identity as a typical couple. Then, three years ago, they were married in Cuba.

Technically, their marriage is illegal in the United States, but Debby wants people to understand that there’s nothing ‘weird’ about her relationship with her half-brother. In fact, she says, it’s the result of a phenomenon called Genetic Sexual Attraction, and it’s surprisingly common.

Genetic Sexual Attraction (GSA), coined by adoption support group founder Barbara Gonyo, is sometimes used to describe the sudden feelings of attraction one might feel towards a sibling or a parent after a family reunion. The Guardian reports that while the frequency of GSA cases is difficult to quantify, some post-adoption agencies estimate that elements of GSA occur in up to 50 per cent of reunions.

GSA might be explained by the ‘Westermarck’ effect — named after sociologist Edward Westermarck —  which holds that people living in close domestic proximity during the early years of life are desensitised to sexual attraction later in life.

Relatives who don’t live together miss out on the the daily events that prevent such attraction from occurring– so that, in effect, the ‘Westermarck’ effect doesn’t have a chance to work in those situations, as Canadian GSA expert and counsellor Dianne Mathes told CBC News.

But for Debby, “It’s nobody’s business as long as it’s consenting adults.

“It’s nobody’s business as long as nobody’s harmed.”