1. The [following] four women: the wife of a man's father, the wife of his son, the wife of his brother, and the wife of the brother of his father, are considered an ervah¹The singular of the term arayot mentioned in the first chapter. for him forever, whether after consecration or after marriage, in the lifetime of their husbands or after their deaths, [even] if they were divorced - with the exception of the wife of one's brother who did not leave a son.²When a man dies childless, one of the brothers of the deceased is obligated to marry his widow to propagate his name. This obligation, yibbum in Hebrew, is described in Deuteronomy, ch. 25, and in Hilchot Yibbum ViChalitzah.

If a man engages in relations with one of these woman during the lifetime of their husbands, he is liable for two [sin-offerings³I.e., if he transgresses inadvertently. If he transgresses intentionally, he is liable for execution, by stoning for relations with his father's wife and his son's wife, and by strangulation for relations with his brother's wife and the wife of his father's brother. (For the latter two transgressions are punishable by kerait and so he receives the penalty for adultery alone.)]: for incestuous relations and adulterous relations, for both of these prohibitions take effect at the same time.⁴See Chapter 17, Halachah 8, and notes where this concept is explained.

2. Therefore a person who engages in relations with his mother who is his father's wife is liable for two [sin-offerings], one because [the woman is] his mother and one because she is his father's wife. [This applies] both during his father's lifetime and after his father's death.⁵Since the Rambam speaks of laws that apply after the father's death, he mentions only two prohibitions. During the father's lifetime, he is liable for a third prohibition: relations with a married woman.

The wives of both a person's paternal brother and his maternal brother are considered an ervah for him. [This applies regardless if he and/or his brother were conceived] in marriage or in a promiscuous relationship.⁶For the prohibition against relations with all blood relatives applies regardless of whether the person was conceived within marriage or outside of it. The wife of the maternal brother of a man's father is, however, forbidden [only] as a shniyah, as explained.⁷Hilchot Ishut 1:5. This is merely a Rabbinical prohibition. Both a person's paternal sister and his maternal sister are considered an ervah for him. [This applies regardless if he and/or his sister were conceived] in marriage or in a promiscuous relationship, e.g., his mother or his father acted promiscuously with others and his sister was conceived promiscuously, as [implied by Leviticus 18:9]: "one born at home or one born beyond [marriage]."

3. The daughter of his father's wife who is his paternal sister is an ervah for him, [as ibid.:11] states: "the nakedness of the daughter of your father's wife, your father's offspring." If, however, a man's father marries a woman and she has a daughter from another man, that daughter is permitted to him.⁸Even if they were raised in the same household like a brother and a sister, marriage between them is permitted. We are not considered with the possible impression such a union might create [Shulchan Aruch (Even HaEzer 15:11)]. She is not "your father's offspring." Behold one is already liable for [relations] with her because she is a sister, why then [does the Torah mention]: "the daughter of your father's wife"? So that one should be liable for this prohibition as well.⁹And thus be required to bring two sin offerings.

4. Therefore a man who engages in relations with his sister who was born to his father's wife in marriage is liable for two [sin offerings]: one because of "the nakedness of your sister" and one because of "the nakedness of the daughter of your father's wife." If, however, one's father raped or seduced a woman and conceived a daughter, one is liable only for having relations with one's sister. For the daughter of the woman who was raped is not the daughter of the wife of one's father.¹⁰This applies even if afterwards, the father marries the woman who he raped or seduced (Minchat Chinuch, mitzvah 196).

5. The sister of his mother is considered an ervah for him. [This applies to both her paternal and maternal sister and applies regardless whether she [was conceived] in marriage or in a promiscuous relationship. Similarly, his father's sister - both his paternal and maternal sister, whether she [was conceived] in marriage or in a promiscuous relationship - is considered an ervah for him.

6. When a person has promiscuous relations with a woman and conceives a daughter with her, that daughter is considered an ervah for him.¹¹The fact that her mother was not married to him is not significant. Although the Torah does not state: "Do not reveal the nakedness of your daughter," the prohibition is of Scriptural origin. Since [the Torah] forbade [relations] with the daughter of one's daughter, it did not mention [the prohibition against] one's daughter. This is not from the words of our Sages.¹²The Rambam's statements touch on an involved issue. In his Sefer HaMitzvot (General Principle 2), he writes that every concept derived through the principles of Biblical exegesis has the power of Scriptural Law. Nevertheless, commandments derived through these principles are not considered as part of the 613 mitzvot, but are instead "from the words of our Sages."

The prohibition against relations with one's daughter, the Rambam states, is not in that category. Although it is not explicitly mentioned in the Torah and Sanhedrin 76a uses different principles of exegesis to derive it, it is not "from the words of our Sages." Instead, it is as if it was explicitly stated in the Torah. From Hilchot Ma'achalot Assurot 9:2, it appears that the Rambam's intent is that since the Torah mentions the prohibition against relations with the daughter of one's daughter, the prohibition against relations with one's daughter is obvious. There is no need for the Torah to mention it. It must be mentioned that many other authorities do not follow the same understanding as the Rambam and consider concepts derived through the principles of Biblical exegesis as fully binding Scriptural Law. According to their understanding, there is no difficulty with the prohibition against relations with one's daughter being considered of Scriptural origin. Therefore a person who has relations with a daughter born of his wife is liable for two [sin offerings],¹³From the Ra'avad's statements, it appears that he does not require a sin offering for relations with one's daughter. The parenthesis are based on the understanding of the Maggid Mishneh. for [relations with] his daughter and for relations with a woman and her daughter.¹⁴As stated in the following halachah.

7. When a person consecrates a woman, her close relatives - six women - become forbidden to him as an ervah forever. This applies whether he consummates [the bond through nisuin] or divorces her, in the lifetime of his wife and after her death. They are: a) her mother, b) her mother's mother, c) her father's mother, d) her daughter,¹⁵When stating this law, Shulchan Aruch (Even HaEzer 15:13) emphasizes that if a man rapes a woman, after her death, he may marry her daughter who was conceived by another man. The Ramah adds that even if the rapist marries her daughter during her lifetime, he is not compelled to divorce her. e) her daughter's daughter, and f) her son's daughter. If he has relations with one of these women during the lifetime of his wife, both [he and she] are executed by burning.

8. If he has relations with one of these women after his wife's death, they are liable for kerait,¹⁶This is based on the Rambam's understanding of Sanhedrin 76b. Rashi, the Ramban, and the Rashba differ and maintain that after the death of the man's wife, he is prohibited against relations with her close relatives, but is not liable for kerait. but they are not executed by the court, as [derived from Leviticus 20:14]: "In fire, he and they shall be burnt." [This implies¹⁷The use of the plural term "they" should not be interpreted to mean that the man's wife should be executed by being burnt to death. For what evil has she committed? Instead, the intent is that only in her lifetime is the death penalty applied (Rashi, Sanhedrin 76b). that only] when both women - his wife and the woman with whom he had relations - are alive, he and the ervah are executed by burning. When both [women] are not alive, there is no execution by burning.¹⁸Sanhedrin, loc. cit., speaks about relations with one's mother-in-law, stating that only when one's wife is alive are these relations punishable by death. Since, however, the prohibition against relations with all the other five women mentioned above is derived from the prohibition against relations with one's mother-in-law, they are bound by the same laws (Maggid Mishneh).

9. Similarly, the sister of his wife is considered an ervah for him until his wife dies.¹⁹For Leviticus 18:18 explicitly states that the prohibition against relations with the sister of one's wife is "in her lifetime." While the wife is alive, even if she is divorced, the man is forbidden to engage in relations with her sister [Shulchan Aruch (Even HaEzer 15:26)]. After his wife's death, however, he may marry her sister. Both her maternal sister and her paternal sister, whether conceived in marriage or promiscuously, are considered as an ervah for him.

10. If a man transgressed and engaged in relations with one of these seven women, whether intentionally or inadvertently, although he and the woman are liable for execution by the court or kerait, he is not forbidden to engage in relations with his wife.²⁰For his wife has not transgressed and there is no reason that she should become forbidden. The only exception is [when he engages in relations with] the sister of the woman he consecrated. In this instance, his wife is forbidden to him, as explained in Hilchot Gerushin.²¹The Rambam is referring to Hilchot Gerushin 10:8-10 which states:

A man consecrated a woman, she journeyed to another country, the husband heard she died, and [then] married her sister. [If,] afterwards, it was discovered that she had not died, he must divorce both women.... Why did they require that the sister of the woman whom the man consecrated be divorced? Lest people say that the [first] kiddushin were given conditionally, [the condition was not fulfilled,] and thus the law would allow marriage to her sister. Since the [first] woman's sister was divorced... the man's first wife is also forbidden to him lest people think that he married his divorcee's sister.

The Maggid Mishneh explains that the Rambam maintains that a divorce is required only in such an instance. If, by contrast, he enters into promiscuous relations with the sister of the woman he consecrated, he may still marry her if she consents. This explanation resolves the protests made by the Ra'avad to the Rambam's statements.

11. When a man engages in promiscuous relations with a woman, the seven relatives mentioned above are not forbidden to him [according to Scriptural Law].²²For the verses (Leviticus 18:18, 20:14) on which these prohibitions are based mention "taking," i.e., marriage. Nevertheless, our Sages²³See Yevamot 97a. prohibited anyone who had promiscuous relations with a woman from marrying one of these seven relatives during the promiscuous woman's lifetime.²⁴After her death, however, there are no restrictions on marrying her relatives (ibid., for the reason for the decree no longer applies. [The rationale is that] the promiscuous woman will come to visit her relatives. He will thus enter into solitude with her. [Since] he is familiar with her, we fear that they will transgress and thus he will engage in relations with an ervah.²⁵For since he is married to one of her close relatives, she is an ervah for him.

Even if a man is merely suspected of relations with a woman,²⁶In his Commentary to the Mishnah (Yevamot 11:1), the Rambam states that this applies when the suspicion is verified. he should not marry one of her relatives until the woman with whom he was suspected of having relations died. If, however, he married the relative of the woman with whom he was suspected of having relations, he should not divorce her.

12. When a person was suspected of having relations with an ervah or a rumor to that effect was circulated, he should not dwell together with her in the same lane or appear in the same neighborhood.²⁷This ruling is derived from the law stated in the following clause of the halachah. The Ra' avad questions the Rambam's deduction, stating that extra stringency is appropriate with regard to one's mother-in-law, but otherwise, there is no need to enforce such a restriction. The Maggid Mishneh states that for that reason, lashes were given only with regard to one's mother-in-law, but agrees with the Rambam's ruling, stating that curbs should be placed on any conduct that may lead to promiscuity. See also Chapter 21, Halachah 27. An incident occurred concerning a man who was rumored [to have engaged in relations] with his mother-in-law and our Sages had him beaten²⁸See Hilchot Sanhedrin 24:5 which states that a judge has the power to subject a person to lashes even if he is not liable according to Torah Law. because he passed by the entrance to her home.

13. When a person has promiscuous relations with a woman and her daughter or a woman and her sister or the like, it is as if he had relations with two unrelated woman. One is considered an ervah because of the other only in the instance of marriage, not in an instance of promiscuity. Similarly, if a man's father, son, brother, or father's brother raped a woman or seduced her, she is permitted to him and he may marry her. [The prohibition involving these individuals] mentions "the wife of" and here there is no context of marriage.

14. When a man's father or son marries a woman, that man may marry her daughter or her mother as we explained.²⁹Halachah 3. A person may marry the wife of his brother's son.³⁰After she was widowed or divorced. A man may marry a woman and her sister's daughter or her brother's daughter at the same time. It is a mitzvah from the Sages for a man to marry his sister's daughter,³¹The Maggid Mishneh explains that a person has a natural affection for his close relatives. Those positive feelings provide a fertile ground of support for the marriage relationship to flourish. as [alluded to by Isaiah 58:7]: "Do not turn away from your own flesh." This law also applies to his brother's daughter.³²Other Rishonim [Rashi, Rabbenu Tam (Sanhedrin 76) differ and maintain that the mitzvah applies only with regard to one's sister's daughter.