To illuminate how within‐family differences in achievement may emerge, this study examined sibling experiences in middle childhood as predictors of sibling differences in college graduation. First‐ and second‐borns from 152 families reported on their experiences with siblings and parents at ages 11.80 (SD = 0.56) and 9.22 (SD = 0.90), respectively, and on their educational attainment at about age 26. Significant childhood predictors of sibling differences in college graduation status included low sibling warmth, fathers’ differential time spent with siblings, and perceived unfair differential treatment by parents. Findings suggest long‐term implications of early sibling dynamics for educational attainment and provided novel insights into families’ role in achievement.