The Torah in Parashat Ahareh-Mot (Vayikra 18:9) forbids “revealing the nakedness of” – meaning, engaging in intimate relations with – one’s sister, even if they share only the same father or only the same mother. If they share one parent, then this prohibition applies. One may not engage in intercourse with one’s half-sister even if she is the product of an illicit relationship. Thus, for example, if one’s father forced himself on a woman, thereby producing a daughter, or if one’s mother had a forbidden relationship which produced a daughter, the brother may not engage in intercourse with that girl. The Sefer Ha’hinuch notes that the Torah in this section also issues a command forbidding relations with “the daughter of your husband’s wife,” even though it already forbids relations with one’s sister. This second verse, the Sefer Ha’hinuch explains, adds a second prohibition, such that one who engages in intercourse with his sister who is also the daughter of his father’s wife is guilty of two violations. This prohibition does not apply to stepsiblings. If one’s father married a woman who has a daughter from a previous marriage, he may marry that daughter. Torah law permits such a marriage, and the Sages did not enact a prohibition forbidding marrying a stepsibling. The prohibition against relations with one’s sibling is binding also upon gentiles, as this is included in the prohibition of “Arayot” – forbidden relations – which constitutes one of the seven Noachide laws. However, whereas for a Jew intercourse is forbidden with a sibling who shares even just a father or just a mother, for gentiles, intercourse is forbidden with a sibling only if they share the same mother. If a non-Jew has a sister with whom he shares a father but not a mother, they are allowed to marry. A gentile who transgresses one of these prohibitions is liable to the death penalty, even if the violation was unintentional, and even if no prior warning was given, as long as the act was seen by two witnesses. The testimony of witnesses is accepted for this purpose even if the witnesses would not be eligible to testify against a Jew. When non-Jews lived under Jewish control, the Jewish authorities were empowered to enforce these prohibitions by administering punishment. This prohibition applies at all times, and in all places. One who intentionally engages in intercourse with a sibling is liable to Karet (eternal excision from the Jewish Nation); Bet Din does not put one to death for violating this prohibition. If one committed this transgression unintentionally, he is required to bring a Hatat (sin-offering).