A TEENAGER who raped his step-sister and asked her to keep a "pinky promise" not to tell anyone has been locked up for five years.

The 17-year-old had smoked cannabis before he forced himself on the schoolgirl at their County Durham home, Teesside Crown Court heard.

Following the night-time attack, he behaved as if nothing had happened, and asked his victim if she wanted a cup of tea and some biscuits.

It was not until two days later that the truth emerged, when the girl confided in a friend and the children's parents were informed.

The lad was taken to Darlington police station by his father and confessed immediately to officers, prosecutor Jo Kidd told the court.

"He was attending to report himself for a sexual assault, and went on to say while he didn't like to use the word 'rape' that's what it had been," said Miss Kidd.

But in a later interview where he had legal representation present, he gave "no comment" replies to questions from detectives on his advice.

Robert Mochrie, mitigating, asked the judge, Recorder Tim Roberts, QC, to give the teenager credit for wanting to own up straight away.

"It was quite clear when he presented himself to the police, it was his genuine intention to make a full confession," said Mr Mochrie.

"The main plank of his mitigation is the issue of remorse. This is something that happened on the spur of the moment and he regretted it immediately."

Mr Recorder Roberts said: "He waited until the cat was out of the bag by another route and he was forced to go to the police station.

"He didn't regret it enough to disclose what had happened earlier, and that the child might need medical attention."

Locking up the teenager, the judge added: "You were in a position of trust and it appears on the night in question you returned intoxicated having taken cannabis and you completely abrogated your responsibility to this child, who was defenceless, who you attacked physically and raped."

The judge told the teenager, who cannot be identified for legal reasons, that he will be on the sex offenders' register indefinitely.