Generally, adult children are perceived to have obligations to support their parents, but now that divorce and remarriage are common phenomena, the question arises to which parent-figures this norm applies. We derive hypotheses on normative obligations towards step-parents and biological parents and the role of co-residential history and divorce. From the perspective of remarriage as an ‘incomplete institution’, we argue that obligations towards step-parents are more ambiguous and therefore more conditional. We collected unique vignette data (N = 4,783) as part of a nationally representative Dutch panel study and predicted norms on adult children’s obligations to provide socio-emotional and practical support using fixed-effects models. We found weaker norms to support step-parents. These are even weaker if there is no co-residential history and/or the step-parent divorced the child’s biological parent, while only co-residence affects norms to support biological parents, and less so than for step-parents. The most ‘disadvantaged’ type of biological parent (divorced, non-residential) is still more advantaged than the most ‘advantaged’ step-parent (married, residential), emphasizing the importance of biology. Analysis of residual variance shows less consensus on obligations towards step-parents than biological parents. It seems that given the absence of clear norms of behaviour, normative obligations towards step-parents are more conditional.