Despite the general belief that sibling relationships are unique in duration across the lifespan and the demonstrated importance of these relationships in childhood, to date, no studies have specifically explored sibling attachment in adulthood. In this study, we examined the sibling relationship from an attachment perspective by exploring the influence of attachment on sibling conflict and cooperation. Results of hierarchical multiple regressions indicated that sibling attachment uniquely influenced conflict and cooperation in the sibling relationship even after controlling for the effects of attachment to mothers, fathers and peers, as well as the reported warmth between siblings. In particular, fearful and dismissing sibling attachment predicted increased conflict and decreased cooperation with siblings. The findings simultaneously highlight the uniqueness of the sibling relationship and contribute important knowledge to the field of attachment, specifically providing some support for the role of siblings as attachment figures.