Based on maternity certainty and paternity uncertainty, one can predict that individuals will channel investment in their kin according to more genetically certain investment options. According to the preferential investment in more certain kin theory, if aunts and uncles have the option to invest in either their sisters’ or their brothers’ children they would prefer to invest in their sisters’ children. However, this question has not been previously explored in contemporary societies with nationally representative data from two generations of aunts and uncles. In our study, we have used data gathered in the Generational Transmissions in Finland project in 2012. The respondents represent older adults (born between 1945 and 1950, n = 1,604) and younger adults (born between 1962 and 1993, n = 1,159). We find that when aunts and uncles have nieces and nephews via both sisters and brothers, they have more contacts with their sisters’ children than their brothers’ children. Thus, the results are in accordance with the preferential investment theory.