Sibling bullying is associated with poor mental health in autistic adolescents. The reasons for this remain unknown. In the current study, we attempted to replicate the existing findings on the direct associations between sibling bullying and mental health in autistic adolescents and expand knowledge by focusing on the indirect associations through self-esteem. We made use of existing data from the Millennium Cohort Study, a nationally representative UK-based birth cohort study. We fitted a mediation model to longitudinal data from a sample of 416 autistic adolescents aged 11, 14, and 17 years old who had at least one sibling. We found that sibling bullying was prevalent in the lives of autistic adolescents, especially in those who were late-diagnosed, had a shared bedroom, and lived in a low-income household. Additionally, increased sibling bullying in early adolescence was a significant predictor of reduced self-esteem in mid-adolescence; in turn, reduced self-esteem predicted poorer mental health and wellbeing in late adolescence. Our findings indicate that sibling bullying in early adolescence may indirectly lead to poorer mental health and wellbeing in late adolescence through a reduction in self-esteem in mid-adolescence in autistic adolescents. We discuss the implications of these findings further.