Inconsistent findings regarding the association between sibship size (i.e., number of children in the home) and children’s theory of mind led us to hypothesize a moderating role for quality of sibling interactions. In line with a parental resource dilution framework, it was expected that coming from a large sibship (3+ children) would be associated with lower ​theory of mind scores in the absence of a cognitively sensitive older sibling. Data were collected from 385 children and their next in age older siblings: at Time 1 children were 3.15 years (SD = 0.27) and their older siblings were 5.57 years (SD = 0.77). Children were, on average, 1.65 years older at Time 2. A longitudinal design, wherein theory of mind (Time 2) was predicted while controlling for earlier theory of mind (Time 1), was used to support directionality of effects. Results indicated that sibship size was negatively related to theory of mind at low but not high levels of sibling cognitive sensitivity. Findings suggest a compensatory role for cognitively sensitive older siblings in large families and highlight the need to consider process-based features of sibships.