A teenager has admitted sexually abusing his younger sister for a year on a monthly basis and sharing video footage of himself raping the vulnerable child.

On Thursday, October 16, the ACT Supreme Court heard it was accepted that the recent abuse had actually lasted closer to two years.

The boy, who cannot be named due to his age, previously admitted two counts of incest and a single count of using a carriage service to transmit child abuse material.

Those charges only reflect two sexual assaults perpetrated by the then-15-year-old.

In court, forensic psychologist Ahu Kocak was cross-examined about her findings that the boy's abuse had been carried out in large part to help him fit in at school.

"The young person's decision-making was skewed towards novelty and external validation rat

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