Despite unmistakable evidence that incest offenders rarely stop with one child, courts routinely fail to protect an incest victim's siblings. Many courts simple deny that a parent's sex act with one child signals risk to others. Nevertheless, even those courts that acknowledge a sibling's risk reach wildly different results when confronting similar cases. Courts are split over whether a parent who molests his stepchild is equally likely to victimize biological offspring or whether a father who violates a daughter will also victimize sons. There is a substantial body of research about incest that can aid courts to better gauge a sibling's risk. Because the first act of incest creates a corresponding risk to some--but not necessarily all--children in the family, Part I of this 2 part series argues in favor of a presumption of risk that offenders may rebut. Part II examines whether a sibling's gender, genetic ties to the offender, or the offender's treatment mitigate the threat and, consequently, should serve as a basis for rebutting the presumption. Ultimately, this article concludes that the law can safeguard children only if guided by substantial evidence of how abusive families function.