This paper examines how the obesity status of different children within the same family is related to a parent or sibling’s obesity. Analyzing results of a 2011 national survey of adults in 10,244 U.S. households, researchers found that the likelihood of childhood obesity varied with the number of children in a household, as well as their gender. In a single-child household, a child was 2.2 times more likely to be obese if a parent was obese. In households with two children, for the elder child, having an obese parent was associated with a 2.3 times greater likelihood of child obesity, and having an obese younger sibling was associated with a 5.4 times greater obesity likelihood. For a younger sibling in a two-child household, having an obese parent was not significantly related to obesity, but having an obese older sibling was associated with a 5.6 times greater likelihood of younger sibling obesity. Within-family sibling obesity was more strongly patterned between siblings of the same gender than between genders. For younger boys in two-child families, obesity was 11.4 times more likely with a male older sibling; and for younger girls in two-child families, obesity was 8.6 times more likely with a female older sibling.