The construction of shared meanings in play, pretense enactment, internal state language, and sibling relationship quality were investigated in 40 kindergarteners with an older (M age = 7.10 years) or younger (M age = 3.6 years) sibling. Dyadic strategies to construct shared meanings (e.g., extensions, building on) were positively associated with frequency of pretense and internal state language. Developmental differences indicated that older dyads used more shared meaning strategies, whereas younger dyads engaged in non-maintenance behaviors (i.e., disruptions to flow of play). Furthermore, firstborn kindergarteners used more non-maintenance behaviors, whereas second-born kindergarteners extended partner's ideas. Findings highlight the sibling relationship as a context for illuminating social understanding and relationship dynamics during pretend play.