Sibling relationships’ effects on social–emotional competence were investigated among elementary school children experiencing changing family structures. The study involved 965 students (ages 10–12 years; 472 boys, 503 girls; N5th grades = 510, N6th grades = 465) in a developed province in China. Sibling relationships and social–emotional competence showed a significant positive association. Sibling rivalry, conflict, and power contrast hindered children’s self-awareness, decision making, and interpersonal skills. Positive sibling relationships (e.g., sibling intimacy) were positively correlated with and predictive of social–emotional competence in children from multi-child families. Sibling intimacy best explained children’s social–emotional competence and predicted the development of social–emotional competence dimensions. Sibling intimacy aspects—pro-sociality, affect, companionship, and admiration—primarily predicted social–emotional competence.