The developmental science literature is riven with respect to (a) parental similar versus different treatment of siblings and (b) sibling similarities and differences. Most methodologies in the field are flawed or confounded. To address these issues, this study employed a within-family longitudinal design to examine developmental processes of continuity and stability in emotional interactions in mothers with their firstborn and secondborn 5-month-old infants (ns = 61 mothers and 122 infants). As independently rated by the Emotional Availability Scales, mothers' observed and coded behavioral expressions of sensitivity, structuring, nonintrusiveness, and nonhostility were consistent in group mean levels between firstborns and secondborns and (largely) between daughters and sons. Neither firstborns and secondborns, nor girls and boys, differed in their responsiveness or involvement of mother. However, mothers' emotional interactions with their firstborn and secondborn children were uncorrelated, as were firstborn and secondborn infants' interactions with their mother. These group-mean consistencies and individual-differences inconsistencies in emotional interactions are discussed in relation to the shared and nonshared lives of siblings in the same family.