Previous research has suggested that first-born infants acquire words faster than their later-born peers (Berglund et al., 2005), but may have some disadvantages in other aspects of syntactic and socio-communicative development (e.g. Hoff, 2006). Here we analyzed infants’ early lexical development alongside their caregiver input from 6-18 months, in relation to how many siblings they have. We find that having more siblings (rather than being first- or later-born) has a gradient and negative relationship with infants’ language development. This affect appears to be manifested in caregiver input: across three different measures of input quality/quantity, disadvantages were found for infants with more siblings. Having a larger number of siblings diminished the quality of the input and led to slower overall lexical development. Implications for language development and learning within dyadic and multi-member contexts are discussed.