Variability within the literature investigating typically-developing siblings of children with an autism spectrum disorder suggests that the quality of sibling outcomes may depend on specific factors. For this study, 42 parents of a child with an autism spectrum disorder and a typically- developing sibling provided data via online questionnaires. Birth order rank of the child with an autism spectrum disorder significantly moderated the relation between externalizing behaviors in children with an autism spectrum disorder and externalizing behaviors in their typically-developing siblings. Children with an autism spectrum disorder and higher levels of behavior problems had typically-developing siblings with higher levels of behavior problems only when the child with an autism spectrum disorder was older. These results provide a hint of clarification about the complex nature of sibling relations, but a great deal more research is needed to further examine outcomes of typically-developing siblings of children with an autism spectrum disorder.