Differential parenting has been shown to be an important correlate and possible cause of positive and negative adjustment of sibling children. However, it is not known whether sibling differences in temperament affect this link between differential harsh parenting and sibling differences in adjustment outcomes. The current study addressed this gap in knowledge. The sample included 92 monozygotic (MZ, 63% female) twin pairs and 137 dizygotic same-sex (DZ, 52% female) twin pairs who had complete temperament survey data collected near the third annual wave in the longitudinal study. Children were 6.09 years old (SD = .69) years old at wave 1. Mothers completed questionnaires, and mother and child were observed interacting during a home visit. Within families, greater sibling differences in conduct problems were statistically predicted by greater differences in harsh parenting exposure between siblings, but not by differences in effortful control. A hypothesized two-way interaction between sibling differences in harsh parenting and differences in effortful control was not significant. Regarding statistical bidirectional “child effects”, greater sibling differences in harsh parenting exposure were statistically predicted by greater sibling differences in conduct problems and greater sibling differences in effortful control. A hypothesized two-way interaction between conduct problems and effortful control was not significant. There was evidence of a bidirectional association between differential harsh parenting and sibling differences in conduct problems. In order to study the proximal family process, it is important to investigate sibling differences using within-family designs. Results can inform parents about how their differential parenting practices may affect child behavioral outcomes, to keep in mind when they parent their children.