Purpose: Limited research has investigated how parenting stress may play a role in sibling relationships. Little is known about co-parenting in buffering against the role of parenting stress on sibling relationships. This study examined these links in a Chinese contest. Methods: A sample of 167 two-child Chinese families were recruited to attend this study. Mothers completed questionnaires assessing parenting stress, co-parenting, parental warmth, and the quality of their children's sibling relationships. Results: Mothers' parenting stress was indirectly associated with positive sibling relationships through the role of maternal warmth. In addition, mothers' parenting stress interacted with their perception of co-parenting to play a role in maternal warmth. Conclusions: These findings provide useful information about the pathway through which mothers' parenting stress influences sibling relationships. In identifying the moderating role of co-parenting on the mediating process of parenting behaviors, we encourage fathers to support the mothers' role in caring for two children.