This study examined the association between observed cognitive sensitivity (CS) during family interactions and children's receptive vocabulary for older and younger siblings. Maternal and sibling CS was considered and associations were explored at the family-wide (between-family) and child-specific (within-family) levels of analysis. The interactions of mothers and 2 children per family were observed when younger siblings were 3.15 years (SD = 0.27) and older siblings were 5.57 years (SD = 0.77). Each dyad (mother-older sibling, mother younger-sibling, and sibling dyad, N = 385) completed a Lego building task, and there were two directional CS scores per interaction (e.g., mother toward older sibling, and older sibling toward mother). Results from multilevel models indicated that younger siblings' vocabulary was associated with the average level of CS for mothers and older siblings, independently. Conversely, older siblings' vocabulary was associated with the average level of maternal CS and child-specific maternal CS. Findings suggest that the relationship between CS and vocabulary operates across the family system and differs for older and younger siblings.