Although several studies have now examined the relations between mother-child and sibling interaction, the role of fathers in the development of sibling relationships is noticeably absent. The present study included assessments of both mother-child and father-child interaction in order to examine the correlates of sibling conflict and cooperation. Home observations of parent-child and sibling interaction and reports of differential parental treatment were obtained for 30 families with 2 preschool children when the firstborns were approximately 6 years old. Earlier assessments of infant-mother and infant-father attachments when firstborns were 12 and 13 months old, respectively, were also available, as were prior laboratory assessments of mothering and fathering when the oldest child was 3 years of age. Results suggested that sibling conflict and aggression were related to high levels of conflict between the mother and the 2 children at 6 years, intrusive and overcontrolling mothering at 3 years, and an insecure infant-mother attachment. Facilitative and affectionate fathering, on the other hand, was associated with prosocial sibling interaction. Early relationship experiences between parents and their firstborn children had an enduring effect on the quality of sibling relationships and interacted with differential parental treatment in predicting sibling relationship outcomes.