The six forbidden relationships for Noahides are: 1) His mother, even if she conceived him in an illicit relationship, since she remains his mother. 2) His father’s wife, as it is written: "A man shall leave his father and mother", which is interpreted to include his father’s wife. This prohibition applies even after the father’s death. 3) A married woman, as it is written: "He shall cleave to his wife", excluding another man’s wife. 4) Homosexual relations, whether with a minor or an adult, as indicated by the exclusion of a male partner from the verse about marriage. 5) Bestiality, as it is written: "They shall be one flesh", implying that only unions capable of producing offspring are valid, thereby excluding relations with animals or birds. 6) Maternal sister, but not paternal sister, because Noahides recognize maternal kinship but not paternal kinship, except for the prohibition of his father’s wife, as seen in Abraham’s words: "She is my sister, the daughter of my father but not the daughter of my mother, and she became my wife."

A Noahide is not liable for relations with a betrothed girl from among their own people, nor even with a married woman unless she has consummated her marriage. If she merely entered the wedding canopy but was not yet intimate with her husband, he is not liable, as it is written: "A woman who has known a man by lying with him." Thus, Noahide law does not recognize betrothal or partial marriage as binding. Additionally, liability only applies for natural intercourse. It also appears that mere sexual contact (ha'ara'ah, minimal penetration) does not constitute a capital offense for Noahides. However, if a Noahide has relations with a betrothed or married Jewish woman who has entered the wedding canopy but not yet consummated the marriage, he is judged by Jewish law, which imposes punishment even for unnatural relations and for minimal penetration. The penalties follow Jewish law: stoning for betrothed women, strangulation for those who entered the canopy but were not yet intimate, and execution by sword for fully married women, as per Noahide law. These punishments apply even without prior warning, with a single witness and a single judge, like other Noahide laws. For Noahides, since they do not have divorce through written documents, a married woman becomes permitted to another man once her husband has sent her away from his home, or if she has left his authority with the intention of not returning.

From what has been stated, it follows that a Noahide is permitted to marry his daughter, and all the more so his paternal or maternal aunt and other relatives. However, there is a strong basis to rule that he is prohibited from marrying his daughter since mother and daughter are considered equivalent in many laws. This interpretation is supported by the Talmudic discussion where Rav Huna states, "A gentile is prohibited from marrying his daughter." The halacha follows this opinion, despite some debate in the Gemara.

Regarding a convert to Judaism, he is considered as a newborn child (ger shenitgayr kekatan shenolad dami), meaning he has no familial ties from his previous life as a non-Jew. Therefore, if his relatives also convert, he is technically permitted to marry them. However, the sages forbade marriage with maternal relatives (but not paternal ones). If a convert had previously married his maternal sister or even his mother as a non-Jew, he must separate from them upon conversion. However, paternal relatives, such as a paternal sister or paternal aunt, remain permitted. Additionally, he may marry his brother’s widow (whether his brother was born before or after his conversion), his father’s widow, or his uncle’s widow, even if these women had married these relatives after converting. The prohibition applies only to maternal relatives. Likewise, if his daughter converted, she remains permitted to him. Similarly, he may marry his maternal aunt’s paternal sister, but not her maternal sister. He is also forbidden from marrying his maternal brother’s widow if she married his brother after conversion. However, if she married him while he was still a non-Jew, she remains permitted. The same applies to his father’s maternal sister. All these laws apply even to a convert who was conceived and born in Jewish holiness (i.e., if his parents converted before his birth). However, the Talmud in Yevamot clarifies that two brothers conceived before their parents' conversion but born afterward are still subject to the prohibition of marrying a brother’s widow. If a convert engaged in relations with his relatives while they were still non-Jews, and he later converted while they remained non-Jews, his actions are considered equivalent to having relations with a gentile, which is not subject to karet (spiritual excision) under Jewish law. If a convert married a woman and her daughter after they had all converted, he must divorce one of them, but he may remain with the other. The same applies to two maternal sisters who converted—he may only keep one. However, if he married a female convert and she died, he is permitted to marry her mother or daughter, since rabbinic prohibitions on such relationships do not apply after death. Similarly, two paternal sisters who convert remain permitted to him. It is thus clear that a convert is permitted to marry all secondary incestuous relations (shniyot) under rabbinic law, such as his maternal grandmother and similar relatives, if they have converted. This discussion is elaborated in Yevamot, in the chapter "Nose’in".