BACKGROUND. How parent and sibling obesity status comparatively shape a child?s obesity is unknown. PURPOSE. To investigate how the obesity status of different children within the same family is related to a parent or sibling?s obesity. METHODS. A national sample of adults in 10,244 American households was surveyed during 2011; data were analyzed in 2012?2013. Of these households, 1,948 adults had one or two children; provided sociodemographic information; and reported on adult and child height and weight, physical activity, and food environment. Logistic regression models were estimated in which the outcome of interest was child obesity status, with parent and sibling obesity as key predictors, adjusting for a range of both adult and child social and demographic confounders. RESULTS. In one-child households, it was 2.2 times more likely (SE=0.5) that the child would be obese if a parent was obese. In households with two children, having an obese younger sibling was more strongly associated with elder-child obesity (OR=5.4, SE=1.9) than parent?s obesity status (OR=2.3, SE=0.8). Having an obese elder sibling was associated with younger-child obesity (OR=5.6, SE=1.9), and parent obesity status was no longer significant. Within-family sibling obesity was more strongly patterned between siblings of the same gender than between different genders, and child physical activity was significantly associated with obesity status. CONCLUSIONS. Considering offspring composition and sibling gender may be beneficial in childhood obesity prevention and intervention.