This study investigates features of interaction between children that are linked to the development of understanding of mind, and asks whether these associations are evident across different relationships. Comparisons are made of the naturally occurring conversations in a sample of 43 4-year-old children, each observed with a sibling and with a friend. Links between features of these conversational interactions and the children's social cognitive development (theory of mind, emotion understanding), language ability and relationship quality are examined. Social cognitive skills were related to successful communication with siblings and with friends, and (marginally) to joint pretend play in both relationships; the relative significance of language for the development of social understanding differed in the two relationships. Language skills and socio-economic background were important only for conversations and pretend play with friends, not siblings.