We studied how individuals’ ethnolinguistic affiliation relates to the ethnolinguistic structure of kinship in contemporary Finland, a society in which Finnish-speaking and Swedish-speaking ethnolinguistic groups have coexisted for centuries and mixed marital unions are common. Using multigenerational data from the population register, we determined how the ethnolinguistic registration of children born in 1990–2015 relates to three generations of ancestors. We created a family tree that links children to their parents, four grandparents and eight great grandparents. Our intention was to both map the ethnolinguistic background of young people and predict a child’s affiliation based on their ancestry. The data revealed that ethnolinguistic affiliation is a more fluid and complex feature than expected when assessed only through child and parental characteristics. We found substantial diversity in ethnolinguistic background within the Swedish-speaking minority group, while most individuals in the Finnish-speaking majority group had a uniform background. We identified three types of bias in the ethnolinguistic affiliation of mixed-origin children: a matrilineal bias, a kinship majority bias and a Swedish ethnic minority bias. The analyses advanced our understanding of how the size of minority groups can shrink even when most couples in mixed unions favour minority group affiliation for their children.