This within-family longitudinal study accomplishes a novel multivariate assessment of socioemotional parenting cognitions and practices in mothers and their sibling children's socioemotional behaviors. Mothers participated with their 20-month-old firstborns and again, an average of 3 years later, with their 20-month-old secondborns (55 families, 165 participants). Continuity and stability in maternal cognitions and practices between the two times, and similarities, differences, and correspondences in siblings' behaviors, are assessed and compared. Maternal socioemotional parenting cognitions were continuous in mean level and stable in individual differences across siblings; maternal socioemotional practices were continuous in mean level but unstable in individual differences. Firstborns were more sociable and emotionally available to mothers than secondborns; first- and secondborns' socioemotional behaviors were largely unrelated. This study contributes to understanding socioemotional domains of parenting and child development, birth order effects, and the shared and nonshared contexts of siblings' environments within the family.