Police dropped their investigation into reports that a stepmother was having sex with her teenage stepson, until the victim came forward four years later.
She has been jailed on Wednesday, March 11 for five years.
The woman, who cannot be named for legal reasons, began having full sex with the boy when she was in her early 30s and he was just turning 16.
She was caught when the teen’s father hid a dictaphone in their home.
Lincoln Crown Court heard the boy’s father became suspicious when he found his partner and son naked in bed together a month after his 16th birthday.
When confronted by her partner, the woman, who was living with the boy at an address near Lincoln, replied: “It’s just a bit of fun, he’s 16.”
The boy’s father remained suspicious and hid a dictaphone in the house which recorded further sexual activity, the court was told.
Police began an investigation in 2013 after the abuse was reported by the boy’s father but his stepmother made no comment during questioning.
The investigation was not pursued by police after the boy said he was 16 when the sexual activity began.
Laura Pitman, prosecuting, told the court: “Oddly it was not taken further by the police despite the legislation.
“Although the age of consent is 16 when it applies to stepmothers the relevant age is 18.”
The woman continued in a relationship with both the boy and his father once the first police involvement had ended.
Miss Pitman said the woman fell pregnant and told her stepson she thought he was the father.
When the child was born, DNA tests in fact showed it was the boy’s father who was the parent.
The victim finally came forward and was interviewed by police when he was aged 20.
He went on to tell police that his stepmother began flirting with him when he was just 15 and full sexual activity began around his 16th birthday.
At the time, his father was in a relationship with his stepmother but living elsewhere.
It began with his stepmother “flirting” with him and escalated to oral and full sexual intercourse in a caravan.
The victim told police that as he got older sexual intercourse occurred on an almost daily basis for two years until he was nearly 18.
In a victim impact statement which was read out in court, the victim, now in his early 20s, described his mental torment at thinking he was a father and having to undergo a paternity test.
“She told me I was the father,” he said.
The victim said he was “manipulated” by his stepmother during his difficult teenage years and that it had damaged his relationship with his own father.
“She made out that she was supporting me and that I did not need anyone else.”
The woman, who had no previous convictions, was examined by a psychiatrist who identified several issues in her past.
Karen Walton, mitigating for the woman, told the court her biggest punishment would be losing contact with her other children while she is in jail.
“She is utterly appalled by her behaviour and still struggles to understand why she acted like this for such a long time,” Miss Walton said.
Now 38, the woman pleaded guilty to three charges of sexual activity with a family member.
The charges included one offence of oral sex, one offence of full sexual intercourse in a caravan and 20 specimen counts of full sexual intercourse.
Passing sentence Judge Julie Warburton told the woman it was an “appalling catalogue of events.”
“You were to all intents and purposes his defacto mother,” the judge said.
The judge also remarked that the first police investigation appeared to have not proceeded following “confusion” over the legislation applying to stepparents.
Judge Warburton told the woman: “Even after the first police involvement, which should have been a shot across your bows, you continued to offend in the same way.”
Judge Warburton added: “You became pregnant. For some time you allowed him to believe he was the father, while simultaneously telling his father the same thing.”
The woman was also ordered to register as a sex offender for life and made the subject of a sexual harm prevention order.