Growing up with multiple siblings might negatively affect language development. This study examined the associations between birth order, sibling characteristics and parent-reported vocabulary size in 6163 Norwegian 8- to 36-month-old children (51.4% female). Results confirmed that birth order was negatively associated with vocabulary, yet exhibited a U-shaped pattern. A data-driven measure of "child-to-caregiver ratio" in the household was developed, in which old-enough siblings--females 1-3 years earlier than males--were considered caregivers for their younger siblings. This measure explained variance in vocabulary better than birth order, and indicates sex-differences in the age at which older siblings contribute to, rather than deplete, available resources. A child-to-caregiver ratio might better capture the interplay between language-learning resources and demands within households.