Research suggests that measures of the family environment show genetic effects when treated as phenotypes in behavioral genetic analyses. We explored this issue as part of the Nonshared Environment in Adolescent Development project using diverse questionnaire measures of parent–child and sibling interactions. The sample consisted of 707 pairs of siblings from 10 to 18 years of age in a novel design (identical and fraternal twins and full siblings in nondivorced families, and full, hall, and unrelated siblings in stepfamilies). Model-fitting analyses yielded evidence for significant genetic effects for 15 of 18 composite measures. On average, more than a quarter of the variance of these environmental measures can be accounted for by genetic differences among children. These results underline the need to investigate the reactive and active organism–environment transactional processes by which genotypes become phenotypes.