This paper uses data on age-adjacent sibling pairs from the National Longitudinal Survey of Youth to test for causal interdependencies between the high school graduation outcomes of older and younger siblings. Even after controlling for observable background characteristics, the graduation probability of an individual whose sibling graduated from high school exceeds the graduation probability of an individual whose sibling did not graduate by a large amount. However, this difference does not measure the causal effect of sibling graduation because of unobserved family factors and genuine simultaneity in the determination of all siblings’ graduation outcomes. To measure the causal effect of sibling achievement on own achievement, I specify models in which sibling achievement is endogenous and estimate these models by two-stage methods using sibling-specific background characteristics as instruments. The evidence indicates that older sibling achievement has a positive causal effect on younger sibling achievement but that younger sibling achievement has no significant influence on older sibling achievement. These results are consistent with a model of intrafamily allocation in which parents learn about child endowments sequentially.