Sibling relationships can be a source of both support and conflict. Due to more severe sibling competition, childhood injuries are predicted to increase as genetic relatedness decreases. Here we use the British Millennium Cohort Study (n = 7,143 children) to analyze the risk of accidents for small children in different types of households. Results show that, after controlling for several potential confounding factors, 3-year-old children who lived in same household with their full siblings had a significantly lower risk of injuries (predicted probability 25.3%) than did children who lived with both their full and half siblings (29.6%) or only with their half siblings (29.1%). We conclude that efforts to prevent child maltreatment should pay attention to sibling relations and family composition.