This study examined the associations between parental differential treatment (PDT), children’s externalizing behavior (EB), and sibling relationships, as well as the intervening effects of children’s perceptions of favoritism, personality, and parents’ self-efficacy (SE). A total of 117 families having a child clinically referred for EB problems were studied. First, the role of PDT and perceived favoritism on EB and sibling relationships was examined. PDT was moderately related to both EB and sibling affection. Perception of favoritism was only predictive of sibling hostility. Second, EB effects on PDT were examined and the mediating role of parents’ SE within this relation was explored. EB predicted higher level PDT in parents and the link between PDT and EB was mediated by parental SE.