Misty Spann, 26, of Duncan, Okla., pleaded guilty in Stephens County District Court and under the deal was sentenced to 10 years of probation.
Authorities uncovered their relationship during a child welfare investigation five months later.
Her mother, 44-year-old Patricia Spann, has pleaded not guilty to incest.
It is reported that misty Spann grew up with her grandmother, as her mother was deprived of parental rights.
Patricia Spann reportedly told the investigator that she had lost custody of her three children, including Misty Spann, when they were children, but she reconnected with them in 2014.
Patricia "looked into" marrying Misty, and thought it was okay.
She also married one of her sons, Jody Calvin Spann, in 2008.
Patricia is expected to appear in court in January.
A woman has been found guilty of incest after marrying her mother. She was also once married to her son, who had the marriage annulled after claiming incest. She forced my sister into this, there's a lot of people that now that.
However, the story gets even stranger when authorities realised this wasn't the first time the mum had been married to one of her children.
"If that's what you want that's on you, but none of us kids want that". Patricia told officers about her history with her children as well as her current relationship, not realising what she was saying was incriminating. When she and Misty were reunited, Patricia allegedly said they "hit it off".
Patricia Spann said that she didn't think the nuptials broke any laws because she was no longer listed as a parent on her daughter's birth certificate.
However, GSA is rare between people raised together in early childhood due to a reverse sexual imprinting known as the Westermarck effect, which desensitizes them sexual attraction.