This study aimed to investigate: (1) the influence of gender, sibling age, and sibling gender on sibling bullying and victimization; (2) the links between personality characteristics, quality of the sibling relationship, and sibling bullying/victimization; (3) the association between sibling and school bullying/victimization, and the direct and indirect associations between personality variables and school bullying/victimization. The sample comprised 195 children (98 boys and 97 girls, aged 10–12 years). Instruments included: a self-report questionnaire for bullying and victimization, the Big Five Questionnaire for Children and the Sibling Inventory of Behaviour. Results highlighted that the presence of an older brother is a risk factor for the emergence of sibling victimization. For both boys and girls, high levels of conflict in the dyad and low levels of empathy were significantly related to sibling bullying and sibling victimization. For males, energy was associated with sibling bullying and indirectly to school bullying; friendliness and high emotional instability were directly associated with school bullying. School victimization was directly associated with emotional instability for both males and females. Finally, both sibling bullying and sibling victimization were associated with bullying and victimization at school. The discussion highlights the role of a multi-contextual approach to understand and prevent bullying.