Pretend play enactment, negotiation, internal state language, and sibling relationship quality were examined in 40 kindergarten-aged children with either an older (M age = 7.10 years) or younger (M age = 3.6 years) sibling. Dyads were identified as engaging in frequent (n = 20) or infrequent (n = 20) pretend play. Results indicated that frequent pretend play dyads engaged in more high-level negotiation, whereas infrequent pretend dyads preferred low-level negotiation strategies. Frequent pretend dyads were more likely to use internal state language, especially during high-level negotiation. Friendly sibling relationship quality was negatively related to pretend enactment, whereas conflict was negatively associated with internal state language. Discussion focuses on the sibling relationship as a context in which to investigate the links between aspects of pretend play, sibling relationship quality, and social understanding.