Two factors have been associated with the quality of sibling relationships: the sex of the children and their family circumstances. Yet the data on each issue are complex. This study examines the sex constellations of sibling pairs in two groups, one of which had experienced a major family disruption (parental separation), with the aim of assessing both influences more fully. Two 1-hour home observations were conducted on 20 preschool sibling dyads of separated parents and 24 pairs in which the parents were married. In both groups the sex configuration of the sibling pairs was important — same-sex pairs seemed to show closer patterns of interaction, and sister-sister dyads were particularly prosocial. These results suggest that previous research showing that older sisters or same-sex pairs interact more are both partly correct. In addition, preschoolers from separated families interacted more. The results suggest that negative life experiences might promote greater closeness between siblings.