This study examined the role of siblings on identity formation in adolescence and emerging adulthood, using a three-wave longitudinal design. Measures of identity formation were filled out by 498 sibling dyads. Sibling effects differed as a function of age and gender configuration within the dyads. Controlled for age, earlier-born siblings reported the most advanced levels of identity formation, and later-born siblings the lowest. Positive relations between siblings' identity and changes in identity of respondents provided support for modeling processes between siblings. The identity of earlier-born same-sex siblings, in particular, tends to be important in influencing identity formation. Contrary to the expectations, differentiation processes between siblings did not appear to influence identity formation. It is apparent from this study that both the gender and birth order of siblings affect whether their own identity formation processes influence those of adolescents and emerging adults.