Within the process of political socialization, the family is of particular importance. Apart from parents, however, little is known about the role of other close family members. The present study examined if siblings affect each other's intergroup attitudes (i.e., intolerance towards immigrants, social dominance orientation). Drawing on a sample of 362 sibling dyads (older siblings: M = 17.77, 53.6% female; younger siblings: M = 13.61, 61.3% female), the results showed that older siblings' intergroup attitudes predicted younger siblings' attitudes, but this effect was moderated by gender. Specifically, older siblings' intolerance and social dominance orientation were only found to affect their younger sisters, yet not their younger brothers. Although younger siblings' intergroup attitudes had no main effect on older siblings, a significant moderation by age indicated that younger siblings affected older siblings' social dominance orientation with increasing age. These moderation effects of age and gender were not mediated by the quality of family relationships. The findings also remained the same when parental intergroup attitudes were taken into account. While siblings were generally identified as an important agent of political socialization in youth, the results also highlight the necessity to further examine the mechanism that either facilitate or hinder sibling effects.