We empirically tested a family systems model's ability to explain variance in psychosocial adjustment among siblings of children with chronic developmental and physical disorders (CDs). The model included the impact of CD severity, family social support, parental mental health, and parent-sibling communication quality. We hypothesized that family social support would moderate the relationship between CD severity and parent mental health, and that sibling-reported communication quality with their parents would mediate the relationship between parent mental health and sibling psychosocial adjustment. We used baseline data from a sibling intervention trial (SIBS-RCT) comprising 288 families with siblings aged 8-16 years and their parents, recruited from health services and user organizations. Parents reported CD severity, parental mental health, and family social support. Siblings self-reported their psychosocial adjustment, including mental health, CD-related adjustment, quality of life, and prosocial behavior. We employed structural equation modeling to test the hypothesized relationships. Higher parent-sibling communication quality was significantly associated with better sibling-reported quality of life and more prosocial behavior. Lower levels of paternal depression were significantly associated with better sibling-reported quality of life and mental health, whereas maternal depression was not. We did not find support for the hypothesized mediating and moderating pathways for family social support and parent-sibling communication. The findings highlight the importance of addressing paternal mental health and parent-sibling communication in promoting sibling psychosocial adjustment. Future studies should use comprehensive, multi-informant approaches and consider the complex interplay of family factors in the context of childhood CD.