A 26-year-old Oklahoma woman has been given 10 years of probation this week after admitting she had a sexual relationship with her 44-year-old mother that even led to marriage.

Misty Velvet Dawn Spann, 26, pleaded guilty to incest in Stephens County, Oklahoma, on Tuesday after the illicit affair was uncovered.

Her mother, Patricia Ann Spann, 44, lost custody of her daughter and two sons when they were children. They were then adopted and raised by their paternal grandmother, reports Daily Mail.

She reunited with Misty in 2014 and says the two of them "hit it off" and started talking about adopting a child together.

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They married in March 2016, after Patricia convinced her daughter that they weren't violating the law by being together.

But indeed they were, and just a few months later, in September 2016, the two were charged with incest after a DHS child welfare worker discovered the relationship.

In Oklahoma, incest is a felony crime punishable by up to 10 years in prison.

Misty decided to take a plea deal, admitting to the relationship in exchange for a lighter sentence.

In addition to spending 10 years on probation, the judge also ordered her to undergo counselling and pay various court fees.

Patricia has pleaded not guilty to the charge, but has not yet appeared in court yet. Her case has been delayed to February so that she could undergo surgery.

What's perhaps most shocking of all is that this isn't the first time Patricia has been caught in a romantic relationship with one of her children.

She also married one of her sons, Jody Calvin Spann, in 2008.

KSWO reports that Patricia told investigators her relationship with her son was not sexual and she married him to prevent him being deployed to the military.

That marriage was annulled two years later by the son, on the basis that it was incestuous.

Patricia and Misty's marriage was also annulled last month at the behest of Misty, who said she was induced "by fraud to enter the marriage".

She says she only consented to getting married after her mother convinced her it was legally OK, saying she consulted three attorneys.

Patricia says she actually did the research herself, and thought that it would be okay since she no longer has the same name as what was printed on Misty's birth certificate.

Misty and Jody's brother Cody Spann told KSWO that his mother manipulated both his sister and brother into marriage.

"I think she's worthless she put my sister into this. She forced my sister into this, there's a lot of people that know it," he said.

"For you to want to put your own daughter through this, what kind of person are you? If that's what you want that's on you, but none of us kids want that, and now you got my sister behind bars because of your choices, why don't you let that sit on you as a mom."

Why would a mother and daughter be sexually attracted?

There's a scientific explanation for why couples like Patricia and Misty Spann get together, despite being related as mother and daughter.

Genetic Sexual Attraction (GSA) is a term that describes the phenomenon of sexual attraction between close relatives, such as siblings, first and second cousins or a parent and offspring who first meet as adults.

The term GSA was coined 30 years ago by American Barbara Gonyo.

She wrote a book about the lust she felt for the adult son she had given up for adoption 26 years earlier. She never acted on her feelings.

However, GSA is rare between people raised together in early childhood due to a reverse sexual imprinting known as the Westermarck effect, which desensitises their sexual attraction.

Experts believe that this effect evolved to prevent inbreeding.