AbstractWe investigated the sibling relationship as a context for cognitive development. Forty preschoolers (ages 5–6) and their younger siblings (ages 2–3) were visited at home. Four games were presented to the older siblings and they were asked (a) to estimate how well their younger sibling will perform on each game and (b) to teach the younger sibling how to use the games. The older siblings' mediating behaviours during the teaching session and the younger siblings' performance on the four tasks were coded. The frequency of mediating behaviours—including attention focusing, amplifying affect and providing meaning, fostering a sense of competence, regulating of the learning process, de‐contextualization, and negative feedback in the form of mocking and laughing at errors, predicted the younger siblings' task performance. The older sibling's accurate perception of the younger child's competence was uniquely predictive of task performance. The highest amount of mediation was observed in older‐brother–younger‐brother pairs, in particular the behaviours of negative feedback and amplifying affect. Results contribute to the discussion on the role of siblings as moderators of cognitive development and are discussed in terms of Vygotsky's cultural–historical perspective on apprenticeship. Copyright © 2002 John Wiley & Sons, Ltd.