As family structure in the United States has become increasingly dynamic and complex, children have become more likely to reside with step- or half-siblings through a variety of pathways. When these pathways are accounted for, more than one in six U.S. children live with a step- or half-sibling at age 4. We use data from the Early Childhood Longitudinal Study-Birth Cohort (N ~ 6,550) to assess the independent and joint influences of residing with a single parent or stepparent and with step- or half-siblings on children’s aggressive behavior at school entry. The influences of parents’ union status and complex sibship status on aggressive behavior are independent. Family resources partially explain the association between residing with an unpartnered mother and aggressive behavior regardless of sibship status. However, the resource hypothesis does not explain the association of complex sibship with aggressive behavior.