Background: Sleep is critical for optimal child development. However, the role of different variables that influence sleep functioning is still debatable. Children with developmental disorders such as Williams syndrome (WS) suffer from sleep problems throughout their childhood. Little is known about sleep patterns of their siblings. Methods: Triplets, one of whom has WS and four typically developing (TD) children matched on age (7.8 years old) and gender participated in the study. Sleep was measured using parental sleep questionnaire and actigraphy. Salivary melatonin and cortisol were measured at three time points during the day. Results: Abnormal sleep patterns, levels of melatonin and cortisol were found in a child with WS and her siblings. Conclusion: These findings indicate that siblings of children with developmental disorders may also exhibit similar sleep problems. Use of multi-level examination should be adapted by paediatricians to devise sleep management strategies for such children and their families.