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Sibling relationships are most individuals' longest lasting relationships, but their development remains understudied. Using a within-family, accelerated longitudinal design with data from mothers, fathers, and two siblings from 201 predominately White, working-, and middle-class families, we charted the development of sibling intimacy and conflict from age 7 to age 30. We also examined structural characteristics (sibling sex, sex constellation, age spacing, birth order) and both person mean (between-person) and time-varying (within-person) links between (a) feminine-typed, expressive personality and (b) maternal and paternal warmth and conflict and sibling intimacy and conflict, respectively, and tested whether sibling age moderated these linkages. Overall cubic changes suggested that intimacy declined from middle childhood through early adolescence and increased through the mid-20s and leveled off, whereas conflict was stable in middle childhood, declined across adolescence, and leveled off in young adulthood, with patterns that varied by sibling sex, dyad sex constellation, and birth order. Furthermore, expressivity was related to both sibling intimacy (positively) and conflict (negatively) at between- and within-person levels; maternal warmth was positively related to sibling intimacy and maternal conflict was positively related to sibling conflict at both between- and within-person levels; and father warmth and conflict were linked to sibling intimacy and conflict, respectively, at the within-person level. Finally, age moderated expressivity effects such that these disappeared in young adulthood and moderated the between-person father warmth effect such that this effect emerged in young adulthood and increased through age 30. Implications for theory and practice regarding sibling and other close relationships are discussed. (PsycInfo Database Record (c) 2024 APA, all rights reserved).
McHale et al. (Mon,) studied this question.