Arthur McElhill, 36, was also bombarding another minor with texts and calls pleading with her to have intercourse with him in the days prior to torching the home in Lammy Crescent.
The heavy-drinking depressive is understood to have killed himself and murdered his partner Lorraine McGovern, 29, and their five children, when he doused the terraced property with petrol and white spirit and lit it in November 2007.
Senior police detective Derek Scott told an inquest into the seven deaths that the teenage girl had told police about her sexual relationship with McElhill.
"He was also corresponding with another underage female with apparent sexual intentions," said the officer.
Two days before the inferno he exchanged 154 texts and 130 calls with the two girls. And on the day before the fire he was on the phone to the girl he had been having the relationship with 34 separate times, in conversations that lasted over two-and-a-half hours.
Giving evidence anonymously from behind a screen, the babysitter, now 18, described how McElhill lured her to an upstairs bedroom to have a look at his computer in July 2007.
On that occasion he kissed her, but a month later he used the same ruse to have sex with her.
"He locked the bedroom door from inside with a key and then pulled the curtains," she recounted in a statement read to court.
The teenager, referred to as witness A, said Ms McGovern and the children had been downstairs when this took place.
Stating that she viewed the sex as consensual, she revealed they had intercourse another five or six times when the rest of family was still in the house and on three or four occasions when Ms McGovern was out.
"The last time I had sexual intercourse with Arthur was about three weeks before the fire," she stated.
Relatives from the wider McElhill and McGovern families, sitting on separate benches, wept openly as she gave evidence.
Bebo
The detective, who officially confirmed that the unemployed farm labourer was the only suspect in the case, also revealed that he had been operating an account on the Bebo social networking site pretending to be his seven-year-old son Sean.
He told the court that prior to the incident McElhill had become aware that neighbours had got to know of his previous convictions for sexually assaulting two young girls.
"He had been confronted during an incident in his house by a female who made it very clear that she had been told during a conversation with two individuals in a public house that he had previous convictions," he said.
The court also heard that McElhill was quizzed by police about an incident in the nearby village of Seskinore two weeks before he set the blaze when an incendiary device had been left at someone's front door.
McElhill had been receiving treatment for depression in the months before the fire and had a history of previous suicide attempts.
The detective said that he believed that his partner Lorraine was not aware of his most recent liaisons with the underage girls.
He gave his testimony at the resumed inquest hearing at Omagh courthouse.
The proceedings had commenced in October but were adjourned after two days after lawyers for the wider McElhill family made an undisclosed legal application.
In October fire crews who found the badly burned bodies of all seven in the house gave evidence.
At that time the court was also played a harrowing 999 call made by the family's eldest child - 13-year-old Caroline - as the flames took hold.
The schoolgirl died, along with her siblings, Sean, four-year-old Bellina, one-year-old Clodagh and 10-month-old baby James.