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Objectives Larger sibships are associated with poorer cognitive and language outcomes but have different impacts on child emotional development. Previous studies have not taken into account sibling age, nor have impacts across multiple neurodevelopmental domains been considered in the same participant group. This study investigated the influence of family size indicators on early childhood cognitive, language and emotional-behavioural development. The effect of sibling age was considered by evaluating these relationships separately for different sibling age categories.Design Prospective birth cohort study. Setting Participants in the Barwon Infant Study were recruited from two major hospitals in the Barwon region of Victoria, Australia, between 2010 and 2013 (n=1074 children). Participants The 755 children with any neurodevelopmental data at age 2–3 years excluding twins and those with an acquired neurodisability. Outcome measures Cognitive and language development was assessed using the Bayley Scales of Infant and Toddler Development, Third Edition, and emotional-behavioural development was measured with the Child Behaviour Checklist for Ages 1½−5. Results Greater household size was associated with a reduced cognitive development score (adjusted mean difference (AMD) −0.66 per extra household member; 95% CI −0.96 to –0.37; p<0.001) without age-specific differences. However, poorer expressive language was only observed for exposure to siblings between 2–6 and 6–10 years older. Having siblings 2–6 years older was associated with less internalising behaviour (AMD −2.1 per sibling; 95% CI −3.1 to –1.0; p<0.001). These associations persisted after multiple comparison adjustment. Conclusions The influence of siblings on early childhood development varies substantially by sibling age and the neurodevelopmental outcome under study. Although family size alone appears important for cognitive devel