We developed a model to account for variation in the quality of sibling relationships across a 4-year span from middle childhood to early adolescence. We tested this model on a sample of 71 families, in which older siblings' ages ranged from 6 to 11 years, and those of younger siblings ranged from 4 to 9 years. We also assessed continuity and discontinuity over time in sibling relationship quality. The descriptive analyses revealed an increase of negative sibling relationship qualities and a decrease in positive qualities. The hypothesized paths involving children's temperaments, interparental conflict, positivity in parent-child relationships, and differential negativity in parent-sibling relationships were supported, accounting for 27% to 34% of the variation in sibling relationship quality.