Infants developing in a cooperative family alliance (FA), characterized by cohesion and mutual support between family members observable during mother-father-child interactions, will likely experience more affect sharing and empathy early in life. Previous studies showed that these experiences might have a positive impact on the development of social cognition, as the development of FA from 3 to 18 months was shown to predict theory of mind (ToM) competences at age 4.5. This study aimed to extend these results to adolescence, as we hypothesized that higher FA in the postpartum would be linked better social cognition skills at age 15 years. We assessed FA during mother-father-child interactions at 3, 9, and 18 months postpartum (N = 49) and adolescents' social cognition skills in a computerized emotion recognition task at age 15 years. Results of growth curve models showed that the stable, but not the changing, components of FA from 3 to 18 months, predicted better emotion recognition-particularly for positive emotional expressions-at age 15 years, when controlling for ToM at age 4.5 years. Results are discussed in light of prior research on the links between early family relationships and children's development from early childhood to adolescence.