Parent and sibling influences on children's development of conflict management strategies were examined. Data consist of naturally occurring, in-home sibling disputes of 37 families at two time points. The siblings were approximately two and four years old at Time 1, and four and six years old at Time 2. Parents' and children's use of conflict strategies that reflect proactive autonomy assertions such as opposition and verbal and physical power were the focus of the current study. Results suggest that parents and siblings play an important role in shaping children's conflict behaviours over time. For instance, parents' reliance on unelaborated opposition to the child, and older siblings' increased use of physical power predicted decreases in younger siblings' ability to assert themselves during conflict. In addition, increased proportions of opposition from a younger sibling was associated with the older siblings using less verbal aggression and using more mature conflict strategies such as justifying their own behaviour to their younger sibling. As sibling conflict is a salient child rearing concern for parents, practical implications are discussed.