According to Metro UK, the 12-year-old boy is said to have repeatedly raped the girl, who was 4-year-old.
The girl eventually told her mother while having her bath and the boy has admitted to raping and sexually touching his sister.
He was found guilty of a second count of rape.
The court was told the boy owned a mobile phone and investigators had found a significant amount of p0rn on it.
Youth offending experts are to carry out weeks of assessment to decide whether he poses a risk to the public and whether he should face jail.
At an earlier hearing in the court, the mother of the girl, who is half sister to the boy, wiped away tears as she described how her little girl had told her.
The girl’s mother, recounting how her daughter told her about the incident, said, “She said to me ‘Mummy I want to tell you something.’ She didn’t normally say things with such an introductory manner.
“She wanted to tell me. It was something important to her. She said it happened while they were playing hide and seek.
“She said he offered her sweeties to do this. She said she didn’t like it but she liked the sweeties.
“I said to her liking sweeties was perfectly all right but he should never have offered her sweets to do something like that.
“She said ‘he made me like Skittles.’ She was clearly cross about that. I said to her ‘why didn’t you tell Mummy?’ and she said ‘He told me not to.’”
The presiding Judge, Teresa Szagun, said a significant amount of p0rnographic material had been found on the boy’s phone.
The judge warned the boy that he faces the possibility of a severe custodial sentence.
The judge said, “I’m sure what she described is accurately what happened to her. The way she spoke was very factual and articulate.
“She was able to tell me about those experiences very well. What she describes was you making her take off her skirt and pants, telling her not to tell, it was a secret, giving her sweets.”
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