Sibling bullying is highly prevalent and has been found to have adverse effects on mental health lasting into early adulthood. What is unknown is what predicts sibling bullying roles (uninvolved, victim, bully-victim and bully). This study aimed to identify precursors of sibling bullying roles in middle childhood using a large sample of 6,838 children from the Avon Longitudinal Study of Parents and Children, a prospective United Kingdom birth-cohort. The relative associations of four sets of precursors: (a) structural family characteristics, (b) parent and parenting characteristics, (c) early social experiences, and (d) child individual differences was assessed before 8 years of age. Structural family characteristics (being the firstborn and having older brothers) and sex (being male) were the strongest predictors of sibling bullying, consistent with an evolutionary model of sibling aggression. Parenting variables, early social experiences, and child individual differences made significant but smaller contributions. These findings may help to identify at-risk families, allowing for appropriate interventions to be implemented from birth.