School engagement has been shown to be a strong predictor for school achievement. Previous studies have focused on the role of individual and contextual factors to explain school achievement, with few examining the role of siblings. This study used data of 451 adolescent sibling pairs from the Iowa Youth and Families Project to investigate the associations between school engagement and achievement in siblings by considering gender composition and birth order. Data were collected in families’ home and obtained for a target child in the 7th grade and for a sibling within 4 years of age. Average age of younger siblings (55% female) was 11.56 (SD = 1.27), while older siblings (49% female) had an average age of 13.92 (SD = 1.47). Using the two-member, four-group actor-partner interdependence model (APIM), results showed that older siblings’ engagement was positively related with younger siblings’ achievement in same-gender sibling pairs, but not in sibling pairs of opposite gender. Younger siblings’ engagement was independent of older siblings’ achievement regardless of siblings’ gender. Implications for parents and professionals suggest that support for older siblings could also benefit younger siblings in same gender pairs.