One of two brothers from Birmingham accused of repeatedly raping their six-year-old sister has told a jury that his mother was "paranoid" about contact between her children.
The 13-year-old boy, who cannot be named, told Birmingham Crown Court he had never touched his sister in a sexual way and also denied "copying" his older brother's alleged behaviour.
Sitting beside a relative in the well of the court, the teenager began his evidence by confirming details of his schooling to his barrister, Jerry Hayes.
The 13-year-old and his brother, who is aged 15, both deny raping and sexually assaulting their six-year-old sister at their home in Birmingham.
Giving evidence on the ninth day of his trial, the younger brother said his relationship with his mother - who alerted social services to the alleged abuse - had deteriorated when he was in Year 7 at school.
"It was normal, but after Year 7 it wasn't very good," he told the court. "I kept getting in trouble at school and at home."
Jurors have been told the boy's mother had spoken to him about inappropriate touching, and alerted the authorities after the six-year-old claimed to have been abused.
During his evidence, the younger boy was asked by Mr Hayes: "Why do you think your mum has been saying these things about you?"
The boy replied: "I don't know. She's probably paranoid, I think. If I was playing with one of my sisters in a room, she would come in and tell me 'You shouldn't, just the two of you, be in the same room'."
The 15-year-old defendant, who is also accused of indecent behaviour with another younger sister, denies two charges of rape and four of sexual assault of a child, while the second defendant denies two counts of rape and two of sexual assault.