Family as the main socialization environment involves mainly parents and siblings. The importance of parents for attitudinal development in adolescence is well-researched. Theoretically, other family members like siblings are also expected to play a role in attitudinal formation. Still, and primarily due to data limitations, siblings are rarely accounted for in empirical studies. In this study, we examine the role of siblings in the development of anti-immigrant attitudes in adolescence (10–17 years), with a particular focus on how sibling effects vary depending on family environment. Using three waves of data from the Czech Household Panel Survey (2016–2018), and mixed multilevel repeated measurement models, we find that siblings’ attitudes towards immigrants are related over time. Moderation effects show that younger siblings are particularly influential under less optimal family conditions. More specifically, we find that younger siblings’ anti-immigrant attitudes influence their older siblings primarily when the family economy is strained and when parents rarely engage in political discussions with their children. Older siblings on the other hand affect their younger siblings’ anti-immigrant attitudes to a similar extent regardless of the family environment.