--- - |2+ Background Sibling aggression is common and often viewed as benign. Although sibling aggression can be harmful for the victims, it may also be a marker of clinical risk for the aggressor. We differentiated typical from atypical levels of perpetration of sibling‐directed aggression among preschoolers, a developmental period in which aggression is a normative misbehavior, by (a) identifying how frequently aggressive behaviors targeted at a sibling must occur to be psychometrically atypical; (b) mapping the dimensional spectrum of sibling‐directed aggression from typical, more commonly occurring behaviors to rarer, more atypical, actions; and (c) comparing the psychometric atypicality and typical‐to‐atypical spectrum of sibling‐directed aggression and peer‐directed aggression. Methods Parents (N = 1,524) of 3‐ (39.2%), 4‐(36.7%), and 5‐(24.1%) year‐olds (51.9% girls, 41.1% African‐American, 31.9% Hispanic; 44.0% below the federal poverty line) completed the MAP‐DB, which assesses how often children engage in aggressive behaviors. We used item‐response theory (IRT) to address our objectives. Results Most aggressive behaviors toward siblings were psychometrically atypical when they occurred ‘most days’ or more; in contrast, most behaviors targeted at peers were atypical when they occurred ‘some days’ or more. With siblings, relational aggression was more atypical than verbal aggression, whereas with peers, both relational and physical aggression were more atypical than verbal aggression. In both relationships, the most typical behavior was a verbally aggressive action. Results were broadly replicated in a second, independent sample. Conclusions These findings are a first step toward specifying features of sibling aggression that are markers of clinical risk and belie the notion that sibling aggression is inherently normative. - Journal of Child Psychology and Psychiatry, EarlyView.