This study explores how changes in sibship composition associated with fertility decline may, in conjunction with entrenched family norms and expectations associated with specific sibship positions, impact marriage rates and further reduce fertility. We evaluate this possibility by focusing on Japan, a society characterized by half a century of below-replacement fertility and widely shared family norms that associate eldest (male) children with specific family obligations. Harmonic mean models allow us to quantify the contribution of changes in both marriage market composition with respect to sibship position and sibship-specific pairing propensities to the observed decline in marriage rates between 1980 and 2010. One important finding is that marriage propensities are lower for those pairings involving men and women whose sibship position signals a higher potential of caregiving obligations, especially only-children. Another is that changes in marriage propensities, rather than changing sibship composition, explain most of the observed decline in marriage rates. We also found that marriage propensity changes mitigate the impact of the changing sibship composition to some extent. However, the limited contribution of changing sibship composition to the decline in first-marriage rates provides little support for a self-reinforcing fertility decline via the relationship between changing sibship composition and marriage behavior.