The source for when a levirate marriage is forbidden TB Yevamot 3

There is general rule that once a woman a forbidden relative (an ervah-עֶרְוָה) in marriage she will always be a forbidden relative the purpose of marriage. The only exception is when a brother dies without children. His brother is commanded to marry the widow. This is a levirate marriage. The Mishnah teaches that if the widow has two reasons why she is a forbidden relative, she not only frees herself from the obligation of marriage, she frees all her rival wives.

“If the deceased brother had two wives, each a so-called rival of the other, and only one wife is a relative of the surviving brother, then the rival wife is also exempt from both levirate marriage and ḥalitza. Moreover, if that same rival wife entered into levirate marriage with a different brother of the deceased, one to whom she is not forbidden, then were this third brother also to die childless, so that the obligation of levirate marriage would again be incurred by the second brother, not only is the forbidden rival wife exempt from levirate marriage and ḥalitza, her new rival wives from her second marriage are also exempt.

“That is to say, any other wife of the third brother is exempt from the mitzva of levirate marriage, as she is the rival wife of that first rival wife, who was exempted from levirate marriage following her first husband’s death due the exemption of her original rival wife. The same principle applies if that second rival wife subsequently enters into levirate marriage with another permitted brother, and so on. In summary, every widow who is exempt from marrying her brother-in-law due to her status as rival wife of a forbidden relative is treated as a forbidden relative herself and is therefore exempt from both ḥalitza and levirate marriage and causes exemption for future rival wives as well.” (Sefaria.org translation)

TB Yevamot daf 3 the Gemara teaches that the sages reached the Mishnah’s conclusion by close reading of the verse and Leviticus and a verse in Deuteronomy. Once they make the connection, the precedent allows the extension of this halakha to all other women. Then the Gemara teaches how we learn that the punishment of marrying a forbidden relative is karet (כָּרֵת). This fact will be important for the understanding of TB Yevamot daf 4.

“The Gemara discusses the halakhot themselves. From where are these matters, that if one’s forbidden relative comes before him for levirate marriage he is prohibited from marrying her or her rival wife, derived? It is as the Sages taught with regard to the verse: “And you shall not take a woman to her sister, to be a rival to her, to uncover her nakedness, with her in her lifetime” (Leviticus 18:18). What is the meaning when the verse states the apparently superfluous phrase: “With her”?

“The baraita explains: Since it is stated with regard to the wife of a deceased brother: “Her brother-in-law will have intercourse with her” (Deuteronomy 25:5), I would derive that when the verse speaks of the mitzva of levirate marriage, it includes even any one of those with whom relations are forbidden, as mentioned in the Torah. Therefore, one derives a verbal analogy: It is stated here, with regard to a wife’s sister: “With her,” and it is stated there, with regard to a levirate marriage: “With her.”

“ The baraita explains the verbal analogy. Just as there, a levirate marriage involves the performance of a mitzva, so too, here, the statement “uncover her nakedness with her” includes the performance of a mitzva, and the Merciful One states in the Torah: “You shall not take.” The phrase “with her” teaches that even in a case where there is an obligation of levirate marriage, the Torah prohibition proscribing forbidden relatives remains in force.

“The baraita continues: And I have derived only that she, his wife’s sister, is exempt from levirate marriage; from where do I derive that her rival wife is also exempt? The verse states: “To be a rival to her” (Leviticus 18:18), which indicates that not only is she prohibited, but so too is her rival wife. And I have derived only her rival wife; from where is it derived that the rival wife of her rival wife is also exempt? The verse states: “To be a rival [litzror],” using the full spelling with a double reish, and not latzor; this indicates that there are several rival wives, one after another.

“And I have derived from this verbal analogy only that the mitzva of levirate marriage does not apply to a wife’s sister. From where is it derived that the same applies to the other women with whom relations are forbidden? You can say as follows: Just as the case of a wife’s sister is specific in that she is a forbidden relative, and this is a prohibition for whose intentional violation, i.e., for intentional sexual relations with her, one is liable to receive karet, and for whose unwitting violation one is liable to bring a sin-offering, and she is prohibited to the yavam in levirate marriage; so too, with regard to all women with whom relations are forbidden by a prohibition for whose intentional violation one is liable to receive karet and for whose unwitting violation one is liable to bring a sin-offering, they are prohibited to the yavam in levirate marriage.

“The baraita continues: And I have derived only the cases of those women themselves with whom relations are forbidden; from where is it derived that their rival wives are also exempt from levirate marriage? You can say as follows: Just as a wife’s sister is specific in that she is a forbidden relative and this is a prohibition for whose intentional violation one is liable to receive karet, and for whose unwitting violation one is liable to bring a sin-offering, and she is prohibited to the yavam in levirate marriage, and her rival wife is likewise prohibited; so too, any woman with whom relations are forbidden and this is a prohibition for whose intentional violation one is liable to receive karet, and for whose unwitting violation one is liable to bring a sin-offering, and this woman is one who is forbidden to the yavam, her rival wife is likewise forbidden. The baraita concludes: From here the Sages stated that fifteen women exempt their rival wives and the rival wives of their rival wives from ḥalitza and from levirate marriage forever.

“§ One might have thought that I should include in this principle even the six women with whom relations are forbidden that are more severe than these, i.e., one’s mother, his father’s sister, and so on, as stated in a later mishna (13b), since they too are forbidden by a prohibition entailing karet. This would mean that their rival wives should likewise be prohibited to enter into levirate marriage with this yavam as forbidden rival wives.

“In response, you can say: Just as a wife’s sister is specific in that she is a forbidden relative and this is a prohibition for whose intentional violation one is liable to receive karet and for whose unwitting violation one is liable to bring a sin-offering, and yet she is permitted to marry one of the brothers but she is prohibited to the yavam in levirate marriage, and her rival wife is likewise prohibited in levirate marriage; so too, with regard to any woman with whom relations are forbidden and this is a prohibition for whose intentional violation one is liable to receive karet and for whose unwitting violation one is liable to bring a sin-offering, and who is permitted to marry one of the brothers but she is prohibited to the yavam in levirate marriage, her rival wife is also prohibited in levirate marriage.

“This excludes the six women with whom relations are forbidden by a more severe prohibition than those, since they may not marry the brothers, i.e., they are forbidden to all of the brothers. One’s mother may never marry his brother, either because she is also that brother’s mother, or because she is his father’s wife. Consequently, their rival wives are permitted, as the halakha with regard to a rival wife applies only due to the brother. In other words, the prohibition against marriage to the rival wife of a forbidden relative is applicable only in instances of levirate bonds. When the levirate bond does not take effect at all, the rival wife is not forbidden.

“The baraita adds: We have learned the warning concerning this prohibition that the yavam may not marry his forbidden relative from the verse: “And you shall not take a woman to her sister, to be a rival to her, to uncover her nakedness, with her in her lifetime” (Leviticus 18:18). From where is the punishment that he incurs if he transgresses and marries her derived? The verse states: “For whoever shall do any of these abominations, the souls that do them shall be cut off from among their people” (Leviticus 18:29).” (Sefaria.org translation)