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.