A developmental cascade model was tested to examine longitudinal associations among firstborn children's aggression, theory of mind (ToM), and antagonism toward their younger sibling during the 1st year of siblinghood. Aggression and ToM were assessed before the birth of a sibling and 4 and 12 months after the birth, and antagonism was examined at 4 and 12 months in a sample of 208 firstborn children (initial Mage = 30 months, 56% girls) from primarily European American, middle‐class families. Firstborns' aggression consistently predicted high sibling antagonism both directly and through poorer ToM. Results highlight the importance of examining longitudinal influences across behavioral, social‐cognitive, and relational factors that are closely intertwined even from the early years of life.