Research on siblings of children with developmental and physical disabilities has emphasized negative influences on siblings' mental health. Yet, such siblings may be more prosocial compared with siblings of children without disabilities, due to care responsibilities and their experiences with their brother's or sister's disability. We compared prosocial behavior between siblings of children with autism spectrum disorder (ASD; n = 47), physical disabilities (n = 42), and siblings of children without disabilities (n = 44) using a multi-informant approach (i.e., child-, mother-, and father-report). Prosocial behavior was measured with the Strengths and Difficulties Questionnaire. Drawing on the theoretical and empirical sibling literature, we also examined whether siblings' internalizing and externalizing difficulties, adjustment to the sibling situation, and communication with parents correlated with siblings' prosocial behavior. Child-reported internalizing difficulties and mother-child communication significantly correlated with mother-reported prosocial behavior. Child-reported internalizing and externalizing difficulties significantly correlated with father-reported prosocial behavior. No significant correlates with child-reported prosocial behavior was identified. When adjusting for siblings' internalizing and externalizing difficulties and mother-child communication, siblings of children with physical disabilities scored significantly higher than siblings of children without disabilities on mother- and father-reported prosocial behavior. Siblings of children with ASD scored significantly higher on mother-reported prosocial behavior. We conclude that prosocial behavior may be a relative strength in siblings of children with developmental and physical disabilities, and that siblings' prosocial behavior may be influenced by type of disability, mental health, and family communication. Interventions targeting siblings' mental health and family communication may be helpful in promoting siblings' prosocial behavior.