--- - |2 This study explored how maternal involvement in sibling relationships and coparenting behaviors were associated with adolescents' sibling conflicts. Adolescents (Mage = 12.25 years; 47.8% boys) and their mothers from 542 families in China participated in this research. Mothers completed questionnaires that assessed their strategies of involvement in sibling relationships, as well as their perceptions of the quality of their coparenting behaviors. Furthermore, adolescents completed questionnaires that assessed sibling conflicts. Results revealed that the mother's positive guidance was negatively related and their authoritarian control was positively related to sibling conflict. A significant interaction was also found between positive maternal guidance in sibling relationships and undermining coparenting behaviors. These findings underscore the unique and interactive effects of mothers' direct involvement in sibling relationships and coparenting behaviors in adolescents' sibling conflicts. - Journal of Research on Adolescence, EarlyView.