Background: Siblings of children with Prader-Willi syndrome (PWS) may be at elevated risk for poor psychological adjustment (Mazaheri, M. M., Rae-Seebach, R. D., Preston, H. E., Schmidt, M., Kountz-Edwards, S., Field, N., Cassidy, S., Packman, Wet al. (2013). The impact of Prader-Willi syndrome on the family's quality of life and caregiving, and the unaffected siblings' psychosocial adjustment. "Journal of Intellectual Disability Research," 57(9), 861-873. ; O'Neill, L. P., & Murray, L. E. (2016). Anxiety and depression symptomatology in adult siblings of individuals with different developmental disability diagnoses. "Research in Developmental Disabilities," 51, 116-125. ). The current study describes psychological distress and symptoms of post-traumatic stress disorder (PTSD) in non-disabled siblings. Method: Fifty-eight siblings and 86 parents participated. Results: Parents reported that almost 40% of siblings had psychological symptoms that exceeded clinical cut-off scores; 58.9% of siblings reported symptoms of PTSD that exceeded diagnostic cut-off scores. Symptoms were significantly related to family organisation and control per parent report and negative affect per sibling report. Conclusion: Growing up with a sibling with PWS may challenge adaptive resources of non-disabled siblings, leaving them vulnerable to psychological distress. Those who care for children with PWS are in a unique position to educate families about the potential vulnerability of non-disabled siblings. We encourage routine screening and support for affected family members, especially siblings.