The aims of this study were to examine family type (nonstep, stepfather, stepmother, and single mother) and sibling type (full- and half-siblings) differences in sibling relationship quality (positivity and negativity), and to investigate links between sibling relationship quality and child externalizing and internalizing problems. The sample included 192 families with a 5-year-old target child and an older sibling. In addition, 80 of these families included a third older child. Mothers and the older siblings (8 years and older) completed questionnaires and interviews regarding conflict and support in their sibling relationships, and parents and teachers reported on each child's social-emotional adjustment. Sibling negativity (conflict, aggression) was highest in single-mother families and full-siblings were more negative than half- and stepsiblings. There was some evidence that sibling antagonism was associated with more child behavioral and emotional problems, but these effects were moderated by family type.