In the much-studied ethical thinking of Paul one aspect has been overlooked: the impact of role thinking. In his letters Paul assigns to the Christians several roles taken from social life, among which the sibling (brother/sister) role has a central position. Paul's frequent and often pointed use of sibling terminology shows that he employs ideas generally associated with social/biological siblingship in order to characterise and influence his co-Christians' attitudes and actions. By doing this, he reinforces and broadens the motivational basis for his ethics, and introduces elements which in important ways shape both its character and its content.