Sibling comparison designs have long been used to assess causal effects of exposures for which randomized studies are impossible and measurement of all relevant confounding is unobtainable. The idea is to utilize the fact that siblings often share a lot of unobserved variables. Therefore, it is proposed that in certain cases, comparing siblings is equivalent to comparing exchangeable individuals, which is the foundation for causal inference based on randomized controlled trials (RCTs). However, this intuition—and the publication of highly important sibling studies—vastly predate modern causal inference theory.