Affective polarization, dislike of political outgroups, is subject to widespread popular and scientific attention and often proposed as an explanation for social ills such as geographic sorting and democratic deconsolidation. Previous research documents the social-psychological underpinnings of affective polarization and largely focuses on `top-down' explanations emphasizing party elites and the media environment. In contrast to these prevailing accounts, we offer a `bottom-up' explanation centering within-family primary socialization. To isolate the role of family environments in cultivating affective polarization, we use a sibling approach, observing affective polarization jointly in pairs of siblings. We do so using data from a well-powered, original, registry-linked survey in Denmark that samples siblings born in 1960 to 1965, and all their children, to retain a large number of siblings and cousins in the children's generation. This unique data set allows for estimating sibling correlations, which isolate the influences of the family environment and regional context that siblings share when growing up. Our results offer a novel perspective on the nature and origins of affective polarization, documenting the extent to which affective polarization is shaped by family-level influences and its stability across family lineage.