This longitudinal study examines family relationships and the adjustment of two adolescent siblings, varying in the degree of biological relatedness, in non-stepfamiles and in stabilized simple and complex stepfamilies. Family relationships and children's adjustment were assessed using interview and questionnaire measures obtained from mothers, fathers/stepfathers, and the two siblings and from observational measures of family interactions in the home. Family type and gender differences in marital relationships, parent-child relationships, and sibling relationships as well as adolescent adjustment were examined. The findings suggest that, even in long established stepfamilies, some differences in family relationships and in adolescent adjustments were found. Few differences, however, between non-stepfamilies and simple stepfamilies were found.