Changes in sibling intimacy and conflict were charted from middle childhood through adolescence, and family structure and relationship correlates of change were examined. Participants were mothers, fathers, and firstborn (M=11.82 years at Time 1) and secondborn (M=9.22 years) siblings from 200 White, working/middle class, 2-parent families. Sibling intimacy was highest for sisters, stable over time for same-sex dyads, and showed a U-shaped change pattern in mixed-sex dyads. Sibling conflict declined after early adolescence at the same time (but at different ages) for firstborn and secondborns. Maternal acceptance covaried positively with sibling intimacy, and father-child conflict covaried positively with sibling conflict over time; fathers' marital love was linked to sibling intimacy in a pattern suggestive of compensation.