A study of 571 families comprising brothers, sisters, a mixture of both and single children found that having a sister in the home led to siblings of either sex scoring more highly on a range of standard tests for good mental health.
They were found to be better at coping with setbacks and more highly motivated than those who grew up with just brothers. They also had more friends and a better social life.
The research, to be presented today at the British Psychological Society's annual conference in Brighton, was conducted by psychologists at De Montfort University in Leicester and the University of Ulster.
Liz Wright, a research fellow at De Montfort, said that the study began after previous research showed that girls with sisters appeared to experience less distress when they encountered trouble in their lives.
"We wanted to see if the positive impact of sisters went farther than just girls and found that it did," she said.
"One of the most interesting findings was the impact of female siblings when parents split up. It seems their natural inclination was to express themselves and encourage other family members to do so as well. There was less distress in broken homes with a sister."
Psychologists have long believed that "emotional expression" at times of upheaval is fundamental to good psychological health.
"Sisters appear to encourage that," Ms Wright said. "However, brothers seemed to have the opposite effect, perhaps discouraging others to talk."
The tests covered how much social support and control over their lives people felt they had, optimism, achievement motivation and ability to cope with setbacks.
This article is from The Belfast Telegraph