The bidirectional nature of siblings’ influence on each other’s cognitive development was examined within the context of naturalistic teaching. The purpose of the present study was to compare 38 sibling dyads from four to seven years of age on measures of teaching strategies, initiation of teaching, learner’s response to teaching, and children’s use of internal state language. Pearson correlations revealed that the older sibling’s age was not related to these variables, while younger siblings’ age was negatively associated with their use of positive feedback. No gender effects were found using a series of t-tests. A repeated measures ANOVA was used to compare older and younger siblings’ teaching strategies. Results indicated that older siblings used more demonstration, positive feedback and negative feedback compared to younger siblings during teaching. Significant main effects were found for children’s use of internal state language, who initiated teaching, and learner response. Overall, both siblings’ referenced more goals than cognitions during teaching. Furthermore, the majority of sibling teaching sequences were initiated by the teacher rather than requested by the learner. Older siblings’ references to cognitions were positively associated with younger siblings’ active involvement and younger siblings’ use of planning was positively correlated with their older siblings’ active involvement. These findings have a number of implications for understanding how siblings contribute to each other’s social-cognitive development.