These are the forbidden unions that stem from ervah, those from the Torah and those from the rabbis. Those that are from the Torah, kiddushin doesn't take effect. Those that are from the rabbis, kiddushin does take effect and you must deliver a get. And likewise a betrothal with a doubtful ervah also needs a get.
A man's mother is forbidden to him according to the Torah, but his grandmother is only forbidden to him from rabbinic law and this law doesn't stop even through the great great great grandmother.
The mother of his mother's father is forbidden to him through rabbinic law, her alone.
The mother of his father is forbidden to him through rabbinic law and this law doesn't stop even through the great great great grandmother.
The mother of his father's father is forbidden to him through rabbinic law, her alone. His father's wife is forbidden to him through the Torah whether or not she's his wife through nisuin (ie fully married) or through erusin (engaged). Whether or not his father is living or whether he's dead or if he divorced her. (Isserles: But if she is his handmaiden she is permitted to him). But if the father took her in force the woman is permitted to him.
His paternal grandfather's wife is forbidden to him according to rabbinic law and this doesn't cease because the wife of Jacob our Father is forbidden for every one of us.
אשת אבי אמו אסורה לו מדרבנן היא בלבד (וי"א דאשת אבי אם אביו אסורה לו) (ת"ה סימן רט"ו):
The wife of a man's father's brother is forbidden to him by Torah law. However, the wife of a man's father's maternal half-brother is only forbidden to him by Rabbinic law.
The wife of a man's mother's father is forbidden to him by Rabbinic law, she alone. There are those who say that the wife of his father's mother's father is forbidden to him.
A man's sister is forbidden to him by Torah law, whether she is his sister from his mother or his father, from wedded or unwed parents. Even if his father had a forbidden relationship and produced a daughter, she is his sister and forbidden to him, unless she is his sister from a female slave or a non-Jewish woman. Rem"a: "It seems to me that a priori she [a man's sister from a mother who is a female slave or non-Jewish] is forbidden to him, because there are those who are satisfied with saying that under Torah law the fetus has the status of the female slave or the non-Jewish woman, but under Rabbinic law it is his child, so therefore one should be stringent a priori." And there are those who say that these words apply to a regular maidservant, but if a man's father had relations with his own maidservant and produced a daughter, she is his sister. Rem"a: Even if he said his intentions were merely for promiscuity, he is not to be believed.
A man is permitted to [marry] the daughter of his father's wife that she has from another man [i.e. his stepsister]. Even if she grew up in the house among the brothers [i.e. her stepbrothers] she is permitted to them, and we are not afraid of marit ha'ayin [an action that appears to be sinful] of people thinking that she is their sister.
A man's daughter, daughter's daughter and son's daughter are forbidden to him under Torah law. But the daughter of his son's daughter and the daughter of his son's daughter and the daughter of his son's son and daughter of his daughter's son are forbidden to him under Rabbinic law, and there is no end to this [i.e. even great-great-grandchildren are forbidden]. And thus it says in Talmud Yerushalmi: Avraham would be forbidden to Jewish women and Sara to all Jewish men. But according to the Rambam, the prohibition does end [after a certain number of generations].
A man's wife's daughter and her daughter's daughter and her son's daughter are forbidden to him under Torah law. Specifically his wife's daughter, but the daughter of his anusah [a woman he married after he raped her] is permitted to him after she dies. Rem"a: Even while she is still alive, if he married her under the condition that he cannot divorce her (See the Gemara about marriages regarding an anusah).
A man's wife's daughter's daughter's daughter and son's son's daughter are forbidden to him under Rabbinic law, and there is no end to this prohibition [i.e. it does not matter how many generations they are removed from each other]. But for the Rambam there is an end.
A man's wife's mother and wife's father's mother are forbidden to him under Torah law. But his wife's mother's mother's mother and father's mother's mother and father's father's mother and father's mother's mother are forbidden to him under Rabbinic law, and there is no end to this prohibition [i.e. it does not matter how many generations they are removed from each other]. But to the Rambam, there is an end.
A man's father's sister and mother's sister are forbidden to him under Torah law, whether she is a sister [to his father or mother] from their father or from their mother.
A man is permitted to the daughter of his father's brother and the daughter of his mother's brother. His father's brother and his mother's brother are permitted to his wife and his daughter.
A man's father's brother's wife is forbidden to him under Torah law. But the wife of his father's maternal half-brother, and the wife of his mother's brother, maternal or paternal, are only forbidden to him under Rabbinic law. And from there on, such as the wife of his father's father's maternal brother (and the wife of his mother's mother's brother) are permitted. The wife of his father's father's paternal brother, and the sister of his father's father whether maternal or paternal, and the sister of his mother's mother are permitted. And there are those who forbid these.
A man's son's wife is forbidden to him under Torah law, and his son's son's wife is forbidden to him under Rabbinic law. There is no end to this prohibition until the [generation of] Yaakov Avinu.
A man's daughter's son's wife is forbidden to him under Rabbinic law, but there is an end to this prohibition.
The wife of his (own) wife's son (his stepson) is pemitted to him, and his wife's son (his stepson) is permitted to his (other) wife.
The wife of his brother, whether (that brother be) from his father or from his mother, whether (that brother was born) from a (legitimate) marriage or from harlotry, is forbidden to him from the Torah.
(Regarding) two stepsiblings who grew up together in the (same) house - each is permitted to marry his friend's wife, and we do not suspect for (the principle of) The Appearance to the Eye which would dictate that (the woman) seems to be his brother's wife (and therefore forbidden on a Rabbinic level although she is actually permitted to him).
One is permitted to his father-in-law's wife, and his father-in-law is permitted to his wife. And there are those who forbid it.
A man is permitted to The wife of his brother's son and the wife of his sister's son. And one is permitted to the daughter of his brother or sister, and it is a Mitzvah to marry her as was stated above, 2.
His wife's sister is forbidden to him from the Torah, all the while his wife is alive; it makes no difference if (the sister) is paternal or maternal, or even if he has already divorced his wife. However, after her (his wife's) death, he may marry her sister. And anyone who gets divorced, even for just a simple rumor, he is forbidden to her relatives (Beit Yosef in the name of RaShB"A s. 13)
One who betrothed a woman and (she) went to a different country and witnesses testified that she died and he went and married her sister and after the fact (it) was revealed that she had not died - he is forbidden to the both of them and the both of them require a divorce from him, as well, all of the other matters listed in the chapter HaIsha Rabbah (lit. The Great Woman, one of the few chapters in Mishna which begins with the word HaIsha, the woman, the word rabbah, great, a discription about the chapter which differenciates it from others with a similar name, Yevamot 87b) and chapter HaZorek (lit. he who throws, Gittin 79b) apply to them. If one of them died, he is permitted to marry her friend (her sister). However if his wife (fully married via nesuin and not merely betrothed) traveled to a different country and witnesses came and testified that she died and he married her sister and then it was understood that his wife did not die - her sister does not require a divorce from him and his (first) wife is permitted to him, and he is permitted to marry the relatives of the second (wife) and she is permitted to marry his relatives and if the first dies he is permitted to the second one. And so (applies) to all 'Arayot (certain forbidden relationships) that one married under the chazaka (halachikally legitimate status-quo assumption) that they were permitted and were then found to be 'Arayot- they do not require a divorce from him because kiddushin (halachikally binding betrothal) does not operate (lit. catch) with 'Arayot. So too all acts harlotry one committed with relatives who are forbidden to him because of his wife do not forbid his wife to him and they (the forbidden relationships) do not become forbidden to him (Tur). However, if that relative would normally come to him because of his wife, (the court) forces him to divorce his wife (al-Fasi there). And why did (the sages) require (the giving of) a divorce to the sister of one's betrothed? Lest (people) say "there was a clause in the engagement and he married her sister legitimately," and once she has exited (the betrothal) with a divorce, her sister, his first betrothed, is forbidden so that (people) do not say "he has married the sister of his divorcée." However when his wife (via nisuin) traveled (to another country), there is no need to suspect that (people) will say "there was a clause in the marriage," because one does not (regularly) render his (legitimate) sexual relations to be(come) promiscuous relations; therefore, all will understand that marriage of the second (wife) was meaningless.
,הַגָּה: וְאִם בָּא בִּזְנוּת עַל אֲחוֹת אִשְׁתּוֹ, וּמֵתָה אִשְׁתּוֹ וְגִיסוֹ אַחַר כֵּן, יֵשׁ אוֹמְרִים דְּאָסוּר לִשָּׂא אַחַר כָּךְ אֲחוֹת אִשְׁתּוֹ, דִּכְשֵׁם שֶׁאֲסוּרָה לַבַּעַל כָּךְ אֲסוּרָה לַבּוֹעֵל, אַף עַל פִּי שֶׁהָיְתָה אֲסוּרָה עָלָיו בְּלָאו הָכִי (פִּסְקֵי מהרא''י סִימָן כ''ח). (וְעַיֵּן לְקַמָּן סִימָן קע''ח): (If) his wife traveled with the husband of his wife's sister to a different country, and they came and told him: "your wife has died as well as her sister's wife," and he married his wife's sister and later his wife and brother-in-law came (back) - his wife's sister requires a divorce from him, no matter if she was married to him through the testimony (lit. mouth) of two witnesses or through the testimony of a single witness, and she is forbidden to his brother-in-law (her first husband) and his (first) wife is forbidden to him, no matter if she was married to him or merely his betrothed. Rem"a: and if he came upon his wife's sister through harlotry and his wife and brother-in-law died afterward, there are those who say that he may not not marry his wife's sister because (even disregarding the familial relationship) "just as she (a wife who had committed adultery) is forbidden to the husband, she is forbidden to the adulterer," even though she was (already) forbidden to him without that (Rulings of MaHaRA"I s. 28). (and see later s.178)
They told him: "your wife died," and he then married her sister and later they told him: "(the first time we had told you that she was dead) she was (actually) alive and (only after he had married her sister) died," the first child (conceived before she had died) is a bastard and the last one (conceived after the wife's death) is not a bastard.
One who had a pilegesh and it was not testified that he betrothed her (via formal kiddushin) - she is permitted to his relatives. However, if there were witnesses that the (pilegesh) woman (herself) said: "he betrothed me before two witnesses," she is forbidden to his relatives. However, if she said: "he betrothed me," simply, and she did not say: "before two witnesses," there is nothing (of relevance to court) in her words.
One who betrothed a woman and the engagement was annulled because it was predicated on a clause and the clause was not fulfilled, or he betrothed one's daughter without her knowledge (what would work if she was a minor) and she was found (to be) an adult - he is permitted to her relatives. And (this applies) specifically if the engagement was annulled without a divorce bill; however, if he gave a bill of divorce - he is forbidden to all of her relatives, as we will explain.