London - Children bullied by their siblings are up to three times more likely to develop a psychotic illness later in life, according to a ground-breaking study.
The eldest in a family is the most likely to pick on their brothers and sisters, while girls are most likely to be victims.
Parents often believe the behaviour is normal and their offspring will outgrow it.
But researchers warn those victimised by siblings several times a week or month are two to three times more likely to develop a psychotic disorder compared to other children.
For those being bullied both at home and in school, the odds are four times as high.
Previous evidence shows adolescents picked on by siblings are twice as likely to suffer depression and self-harm.
But the latest study led by Warwick University is the first to look at psychotic symptoms, such as schizophrenia, hallucinations and delusional thinking.
Nearly 3 600 children were questioned about sibling bullying aged 12 and about psychotic symptoms aged 18. Of these, 664 were victims, 486 were bullies and 771 were both a bully and a victim of bullying in the family.
By 18, a total of 55 had a psychotic disorder, reported the journal Psychological Medicine.
Senior author Professor Dieter Wolke said bullying by siblings "can lead to self-blame and serious mental health disorder as shown here for the first time".