Observed face-to-face interactions between 39 mothers and their first and second-born two-month olds to evaluate within-family differences in maternal affective behavior toward siblings. Found that mothers were more positive with second-borns than firstborns, and second-borns were more positive than firstborns. Siblings' affective behaviors were unrelated, but maternal positive affect was moderately stable between siblings and correlated with each infant's affect. (Author)