- Growing up with at least one girl in the family also makes people more able to cope with their problems, according to the study.
Daughters tie loved ones closer together and encourage them to communicate their emotions more effectively, the researchers believe.
Prof Tony Cassidy, from the University of Ulster, who carried out the study with researchers from De Montfort University in Leicester, said that having a sister helped to promote good mental health.
He said: "Sisters appear to encourage more open communication and cohesion in families. However, brothers seem to have the alternative effect. Emotional expression is fundamental to good psychological health and having sisters promotes this in families."
Girls who had sisters also tended to be more independent and keen on achievement, according to the findings.