A boy has been placed into local authority care and separated from his younger sister and brother after social services staff raised concerns that he had been "over-involved" in parenting them.
The behaviour of the boy, who turned 12 last year, posed "difficulties" - and the younger children had said they were afraid of him, a family court judge heard.
Judge Peter Greene concluded that all three should be taken from the care of their parents and placed into long-term foster care.
And he decided the older boy should live apart from the younger children - a sister who turned nine last year and a brother who was seven last year.
The judge had analysed the case at a private family court hearing in Peterborough, Cambridgeshire.
Detail has emerged in a ruling.
He said the children could not be identified.
Judge Greene said the boy had been diagnosed with attention deficit hyperactivity disorder and oppositional defiance disorder and had been prescribed medication.
"He is assessed as being a very anxious child," said the judge.
"(He) has extensive needs."
The judge added: "He has been over-involved in parenting his younger siblings.
"They have occasionally expressed a fear of him."
Social services staff had undertaken a "sibling attachment assessment" in view of the "difficulties" posed by the boy's behaviour.
They had recommended he should be separated from his younger siblings.
Judge Greene said the children's mother was 27 and their father 56.
The couple had met when she was in her mid-teens and he was 43 - and she had complained of being "abused and controlled" by him.