Links between sibling relationships, mother-partner, and parent-child relationships were studied in a longitudinal community sample of 3681 sibling pairs. Individual differences in sibling relationship quality were related to mother-partner affection and hostility assessed 4 years earlier, to contemporary parent-child negativity, and to indices of social adversity. Evidence for both direct and indirect pathways (via parent-child relations) linking mother-partner and sibling relations were found. Comparisons of prediction for non-stepfamilies and stepfather families showed similarities in patterns of association, but also differences: In stepfather families, mother-partner hostility was unrelated to parent-child negativity and sibling relationship quality. Both positivity and negativity towards young siblings decreased with the age of older siblings, and older sisters were more positive than older brothers.