This study examined longitudinal links between incongruence in mothers' versus fathers' differential treatment of adolescent-age siblings and parents' marital quality. Multilevel models including 200 families, over four waves, spaced across 6 years tested whether youth perceptions of incongruence in differential intimacy and conflict predicted trajectories of mothers' and fathers' reports of marital conflict and satisfaction and vice versa. Analyses showed that changes in interparental incongruence covaried longitudinally with changes in marital quality and that these linkages became stronger over time. These results extend previous cross-sectional research with younger children and are consistent with theories regarding family alliances and coparenting. Discussion focuses on the reciprocal relations between incongruence in parenting and marital quality as an important aspect of family systems.