The Torah in Parashat Ahareh-Mot (Vayikra 18:17) forbids having intimate relations with “Isha U’bitah” – a woman and her daughter. This means that if a person marries a woman, he may not then ever marry that woman’s daughter, even though the daughter was fathered by a different man. This prohibition applies only to the daughter of a woman whom one marries. However, if a man had an illicit relationship with a woman, out of wedlock, then Torah law does not forbid him from marrying her daughter from a different man. Since the man never married the mother, the daughter is not forbidden for him. The Sages, however, enacted a provision forbidding one from marrying all immediate family members of a woman with whom he had an illicit intimate relationship. The reason is that if one marries a family member, it is likely that the woman with whom he engaged in relations will occasionally visit. And since she and this man are comfortable with one another, having already been intimate together, they might have an adulterous relationship. The Sages therefore forbade marrying a family member of a woman with whom one had once illicitly engaged in relations. The prohibition of “Isha U’bitah” applies in all places and in all times. If one had relations with his wife’s daughter from a different marriage during the mother’s lifetime, then both he and the daughter are liable to Serefa (execution by burning). If this relationship took place after the mother’s passing, then the punishment is Karet. If this prohibition is transgressed unintentionally, then the violator must bring a Hatat (sin-offering).