Sibling Complexity Across Childhood: A 16‐Year Follow‐up of the 1988 and 2000 Birth Cohorts in Finland

Objective This study quantifies sibling experiences across childhood by birth cohort and maternal education. Background As family structures evolve, sibling complexity—the presence of half and stepsiblings—is becoming more common. Yet, little is known about how sibling constellations unfold across childhood, vary between cohorts, or differ by social background. Method Annual register data of full Finnish birth cohorts 1988 ( n = 64,597) and 2000 ( n = 56,413) were used to identify all full, half, and step‐sibling constellations from birth to age 16. Sequence analysis was employed to create relative frequency plots and calculate the number of and time spent in different sibling constellations. Results By age 16, 35% of children in the 2000 cohort had experienced sibling complexity, nearly 20% more than the 1988 cohort. The most common experience of sibling complexity was growing up with older full and half siblings. Trajectories with stepsiblings were less common, and having both half and stepsiblings remained rare. Children of basic‐educated mothers experienced more sibling complexity and instability, with greater increases over time than children of more educated mothers. Conclusion Increases in family complexity extend into changing sibling kinships, but the experiences of sibling complexity vary considerably by social background. Research and policy should acknowledge the growing share of families navigating diverse sibling constellations and attend to both the causes and consequences of these evolving experiences.