This study built on research on sibling influences to assess potential bidirectional effects of older and younger siblings' risky behaviors on one another's risky behaviors; our longitudinal design allowed us to test these effects when siblings were at about the same chronological age, at different points in time. We also tested whether the strength and/or direction of effects of siblings' risky behaviors changed from middle adolescence to young adulthood. Reports of risky behaviors (i.e., deviant behaviors and excessive alcohol use) were provided by firstborn and secondborn siblings from up to 201 families on five occasions spanning 10 years. In general, accounting for known covariates, multilevel models revealed bidirectional sibling effects and some evidence that secondborns' risky behaviors were stronger and more consistent predictors of firstborns' behaviors than the reverse. Sibling influence generally declined with age and sibling effects were not moderated by gender constellation. Findings indicate that both older and younger siblings are important socializers of risk behaviors across adolescence and continue to shape each other's alcohol use into early adulthood.