Based on the expectancy-value theory, cultural microsystem model, and family systems theory, this study aimed to understand (a) the extent to which Latinx older siblings’ support predicted the high school science motivational beliefs of their younger adolescent sibling, and (b) whether these relations varied based on older siblings’ familism values and gender. This study included data on 104 Latinx adolescents in 9th grade and their older siblings. Quantitative regression analyses indicated that older siblings with high familism values were more likely to provide higher support. Older siblings’ support did not predict science self-concept or task value for all youth; however, this relation was significant when the older sibling’s familism values were high. Older sibling gender was unrelated to any indicators in this study. These findings suggest that familism values may play a significant positive role in Latinx sibling dyads, with older siblings who are more connected to their family serving as a significant resource to promote science motivation among their younger siblings.