This study examined 32 children's (M age = 1;8 years) engagement in joint attention (JA) and the relation between JA and vocabulary size across mother-child (MC) and mother-child-sibling (MCS) contexts. In the MCS context, mothers engaged in JA more with one child than both children; they engaged in less JA with target child than they did in the MC context. JA style was generally unrelated across the contexts. Coordinated JA and children's vocabulary were significantly related only for the MCS context. Findings suggest the number of social partners influences JA dynamics and multi-child contexts can be positive language learning environments.