Background: Smith-Magenis syndrome (SMS) is a neurodevelopmental disorder that affects approximately one out of 25 000 births worldwide. To date, no research has been conducted to investigate how having an individual with SMS in a family is a positive or negative influence on siblings. Methods: To investigate this question we conducted a study involving 79 siblings and 60 parents of individuals with SMS to assess perceptions of how having a sibling with SMS positively and negative influence siblings' behavioural traits. Results: Our findings show that age of siblings of individuals with SMS was associated with a significant increase in positive behavioural traits and a significant decrease in negative behavioural traits. Additionally, siblings who perceive benefits from having a sibling with SMS demonstrate significantly more positive behavioural traits and significantly fewer negative behavioural traits. Parents accurately assess the changes in sibling behavioural traits with age, and parents who perceive their child as having experienced benefits from the sibling relationship report that siblings demonstrate significantly more positive behavioural traits and significantly fewer negative behavioural traits. Conclusions: Our research shows that although individuals experience difficulties as a result of having a sibling with SMS, overall, siblings tend to fare well and parents appreciate both the positive and negative behavioural effects that result from having a sibling with SMS. (Contains 3 tables and 4 figures.)