Integrating situated expectancy-value and family systems theories, the current study tested the extent to which Latinx adolescents' 9th-grade school-related science conversations with parents and older siblings/cousins positively predicted their 10th-grade science ability self-concepts and task values. We also tested whether these links were moderated by who primarily initiated the conversations (i.e., adolescents, family members, or both). We used two-wave, multi-reporter survey data from 104 Latinx families, consisting of triads of parents, older siblings/cousins, and adolescents (89% Mexican-descent, 40% female; M[subscript age] = 14.53 years). Partially supporting our hypotheses, parent-adolescent school-related science conversations predicted adolescents' 10th-grade science ability self-concepts. Moreover, the links between parent-adolescent conversations and science ability self-concepts and task values were positive and significant when parents more frequently initiated conversations than adolescents. Similar but weaker associations were found for sibling/cousin-adolescent school-related science conversations. These findings underscore the motivational benefits of family members initiating school-related science conversations with Latinx adolescents.