Valid information on early social-emotional competence is essential to diagnose, treat, and prevent behavioral problems in children and adolescents. Particularly in young children, social-emotional competence is frequently measured using parent and teacher ratings that frequently exhibit low agreement. Therefore, the present study on n = 532 three-year-olds (47% girls) examined whether sibling status might explain discrepancies between the two informant groups. First, multi-trait multi-informant analyses explored the construct validity of a short measure of three facets of social-emotional competence. Then, group comparisons evaluated the size of the observed method effects for only children and children with siblings. Results showed low convergent validity between parent and teacher ratings for aggressive behavior, cooperative behavior, and emotional self-regulation. Sibling status in the family contributed little to the observed discrepancies between parents and teachers. Thus, a comprehensive assessment of social-emotional competence in children requires a multi-informant approach to capture the construct breadth.