Girls who receive higher quality fathering engage in less risky sexual behavior (RSB) than their peers. Previous research identifies higher levels of parental monitoring/knowledge and reduced affiliation with deviant peers as potential mediators of this observed fathering effect. Although paternal investment theory posits a causal effect of fathers on daughters’ RSB, and on the intervening processes that mediate this effect, these relations could arise from genetic or environmental confounds. To address this limitation, we employed the genetically- and environmentally controlled differential sibling-exposure design (N = 101 sister pairs; ages 18–36), which retrospectively examines the effects of differential sibling exposure to family disruption/father absence and quality of fathering. Consistent with a causal explanation, differences between older and younger sisters in the effects of fathering quality on parental monitoring and peer RSB were greatest in biologically disrupted families when there was a large age gap between the sisters (thus maximizing differential exposure to fathers), with greater exposure within families to higher quality fathering increasing parental monitoring and reducing affiliation with sexually promiscuous peers. No such differences were found between older and younger sisters in families with little or no differential exposure to fathers (e.g., biologically intact families) or in response to differences in mothering quality. Taken together, these findings suggest that higher quality fathering may decrease daughters’ engagement in RSB by increasing the amount of parental monitoring that they receive and decreasing their affiliation with peers who promote RSB.