Background: Family composition and household dynamics, both in early and in later life, influence individual health and longevity. Both positive and negative effects can be expected in terms of sibling size and composition. On one hand, siblings compete with each other, which may lead to resource dilution and increased adult mortality risks. On the other hand, siblings protect and care for each other, which may have a positive impact on longevity.

Objective: To investigate the way in which sibling composition (with respect to sibship size, sex, and birth order) in the family of orientation and the proximity of siblings in later life relates to adult mortality risks at ages 50+.

Methods: Life courses of 258 men and 275 women from the Antwerp COR*-database were 'reconstructed' and analyzed by way of event history analysis using Gompertz shared frailty models.

Results: Being higher in birth order related to significantly higher mortality risk after age 50 for men. Having older brothers, particularly those present in later life, was associated with very high excess mortality risk for both sexes, though men were more strongly disadvantaged. Having (more) younger sisters present at RP (research person) age 50 was related to significantly lower relative mortality risk for women.

Conclusions: Our findings highlight the complex relationships between sibling and gender dynamics and mortality risk in later life. Evidence of a lasting impact of sibling competition on mortality risk over age 50 is found; and competition is only replaced by solidarity in critical times (e.g., widowhood), wherein older sibling presence dissimilarly impacts different social groups.