In this study, the importance of family factors in the development of posttraumatic stress reactions in children after trauma is studied by comparing siblings and nonsiblings who experienced the tsunami in Southeast Asia in 2004 as tourists. Thirty-eight sibling pairs aged 6–17 years were interviewed using the UCLA Posttraumatic Stress Disorder Reaction Index. The sibling differences were similar to differences found in randomly selected children in the same sample, indicating that family influences may not be as important as anticipated. The results are an important contribution to our understanding of what contributes to and influences the development of posttrauma reactions in children. They also suggest the importance of conducting individual assessments even after mutually experienced trauma.