Background: Young siblings' antisocial behaviour is common yet its impact has received relatively little research attention. Methods: We examined trajectories of antisocial behaviour for a socially diverse sample (n = 99, 58 boys and 41 girls) who were filmed with their older siblings (52 boys and 47 girls) at ages 3 and 6 and with unfamiliar peers at age 6. Latent growth models were used to analyse three indicators of antisocial behaviour (refusal to share/interact, bullying and harming). Results: The average trajectory of antisocial behaviour towards siblings was stable and particularly high for boys with brothers and for children of mothers with no educational qualifications. Sustained and escalating antisocial behaviours towards siblings predicted bullying and refusals to share/interact with unfamiliar peers, independent of associations with concurrent antisocial behaviour towards sibling. Conclusions: These findings highlight the importance of a developmental perspective when examining antisocial behaviour between young siblings.