Individual differences in longitudinal trajectories of children’s social behaviors toward their infant sibling were examined simultaneously across multiple social dimensions: Positive engagement (moving toward), antagonism (moving against), and avoidance (moving away). Three distinct social patterns were identified: (C1) positively-engaged (n = 107, 50%); (C2) escalating-antagonism (n = 90, 42%); and (C3) early-onset antagonism (n = 16, 8%). Children in the positively-engaged class had high levels of positive engagement with their infant siblings, coupled with low levels of antagonism and avoidance. The escalating-antagonism class was positively-engaged in sibling interaction with a steep escalation in antagonistic behavior and avoidance from 4 to 12 months. Children in the early-onset antagonism class displayed the highest level of antagonistic behavior starting as early as 4 months, and became increasingly avoidant over time. A path model, guided by a Process × Person × Context × Time model, revealed that low parental self-efficacy heightened by parenting stress and children’s dysregulated temperament was directly related to the escalating-antagonism pattern. Punitive parenting in response to children’s antagonistic behavior increased the likelihood of being in the early-onset antagonism class. Together, the results highlighted heterogeneity in the earliest emergence of sibling interaction patterns and the interplay of child and parent factors in predicting distinct sibling interaction trajectory patterns.