Extant research documents how siblings’ relationships develop from childhood through adolescence; yet, we know little about how sibling relationships change in young adulthood. Rooted in life course theory, this 2-wave longitudinal study investigated changes in sibling closeness and conflict, and the roles of life transitions and sibling similarity in life stage. Participants included 273 young adults from 180 families who reported on 340 sibling relationships (Time 1 M age = 24.45, SD = 5.33; Time 2 M age = 30.23, SD = 5.33). Multilevel repeated measures analysis of covariance indicated that, on average, siblings’ perceptions of conflict declined over the course of young adulthood. Additionally, patterns of change in closeness and conflict were linked to life transitions surrounding coresidence, parenthood, and similarity in employment. The discussion addresses findings in regard to life course theory and similarity in life transitions.