Rachel and Shawn are devoutly religious, and even point to the Bible as a precedent for their love.
"If you take Adam and Eve," Shawn says, "where did we all come from if there wasn't incest?"
Genetic sexual attraction is not a phenomenon that is exclusive to sibling couples. According to Barbara Gonyo, a 70-year-old grandmother in the Midwest, it can also occur between parents and their estranged adult children.
Gonyo was only 15 years old when she discovered she was pregnant in the early 1950s. Her parents forced her to give up her baby for adoption, and she remembers how traumatic it was to watch the nurse at the hospital immediately take her baby son away from her.
"'Say goodbye to your mommy now'… was all she said," Gonyo says. "It's like somebody put a rock in my heart. It was horrible."
As the years passed, Gonyo never stopped wondering about the son she lost. She sought him out, and almost 30 years ago, when he was 26, they were reunited. The first thing Barbara noticed was how much her son looked like his father, the first love of her life.
Gonyo's son made it clear to her that he did not want to call her "mom" out of respect for his adoptive mother. As Barbara realized she couldn't be his mother, her emotions began to shift.
"If I can't be his mother, who can I be?" she asked herself. "So I started to feel more romantically towards him that I did motherly."
Gonyo was sickened by her feelings and feared she was crazy. But she soon began to understand her feelings to be genetic sexual attraction. She believed she had feelings of attraction because she had missed out on bonding with her son.
Joe Soll, who wrote a book called "Adoption Healing," says GSA often happens in people separated by adoption.
"They want to be close and hug because they haven't had the relationship for 20 or 30 years," he says. "And that hug can turn into something else."
Not everyone believes GSA is a legitimate phenomenon. Adoption expert Adam Pertman says genetic sexual attraction is no more common a phenomenon than incest.
"It happens in adoption just like it happens in biologically formed families," he says. "It's rare in adoption just like it's rare in biologically formed families. If we act on it, that's called incest, and we don't think that's a great idea for lots of good reasons."
Gonyo agrees that acting on those feelings would have been a horrible mistake. Even though she had an intense attraction toward her son for 15 years, her son never reciprocated, and she never acted on desires.
"Emotionally," she says, "you are the adult, and they are the child. If you go through with the sexual act, then I hold the parent responsible, not the child."
But half siblings Rachel and Shawn don't feel the need for restraint. They couldn't be happier that they finally met and fell in love.
"I apologize to everybody out there that's a nonsibling couple, but you will never have what we have ever, ever, ever," says Rachel. "It just simply cannot exist outside of what we have."