The 12-year-old boy is said to have repeatedly raped the girl, who was four at the time.
She eventually told her mother while having a bath and the boy, now 13, has admitted raping and sexually touching his sister.
He was found guilty of a second count of rape after a hearing at Brighton Youth Court.
The court was told the boy owned a mobile phone and investigators had found a significant amount of porn on it.
The court was told the boy owned a mobile phone and investigators had found a significant amount of porn on it.