It has been argued that an increase in the number of siblings means that there are fewer parental investments made per child (resource dilution hypothesis). Yet, these studies are mostly based on biological two-parent families in which it can be assumed that parental resources are distributed more or less equally across siblings. This assumption does not hold in the complex family structure of a stepfamily where not all siblings are biologically related and living in the same household. Since more and more children are growing up in stepfamilies, a distinction between sibling types (full/half/step) is needed. In this study, we focus on the dilution of nonmaterial resources (i.e., parental involvement) and use the OKiN dataset to examine paternal (n = 1,077) and maternal (n = 1,369) resources separately. In this way, this study provides the first comprehensive overview of the dilution of parental resources in stepfamilies. While maternal resources are mostly diluted by full siblings, paternal resources are diluted by all sibling types (full, half, and step). We reflect on the implications of these results for the literature on sibship size as well as the literature on social mobility.