A vast body of research focuses on the role of parents in athlete development; however, little is known about developmental influences of siblings. In general, family dynamics (i.e., patterns of relating or interacting among family members) have yet to be investigated in youth sport contexts. This study examines how family dynamics and the individual roles of parents and siblings influence the development of Canadian interuniversity student-athletes over time. Participants included four male and six female student-athletes. Each participant took part in a qualitative retrospective timeline interview. All data was subjected to a thematic analysis. Results indicate that siblings and parents play separate yet intricately connected roles in athlete development throughout childhood and adolescence. Overall, participants described a cohesive family unit built on shared values and joint participation in sport activities. They described stable and dynamic forms of support from their parents over time, and positive and negative sibling influences. These findings offer valuable insight into the dynamic nature of parent and sibling relationships with athletes in youth sport and beyond, as well as how these relationships operate in the broader family environment to optimize (and, at times, hinder) athletic development.