Families are theorized to influence adolescents’ participation in skill-based after-school activities, but research has focused on the role of parents while neglecting the role of siblings. Siblings might be especially critical for Mexican-origin youth, the fastest growing youth population in the United States, due to a high value of family as well as having a larger number of and spending more time with siblings compared with other groups. This mixed-method study of 34 Mexican-origin seventh-grade adolescents provides preliminary evidence for how siblings might influence adolescents’ skill-based organized and informal after-school activities. Qualitative findings suggest that siblings influenced adolescents’ activities through eight unique behaviors (i.e., support, provider of information, role modeling, comparison, negativity, babysitting, transportation, and activity co-participation). Follow-up quantitative analysis suggested some sibling behaviors were more likely to be mentioned based on sibling characteristics, individual characteristics, and neighborhoods. The study highlights new theory, emerging future directions, and ways to promote positive after-school activities via siblings.