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.