Yevamot 40B

הוא אסור באמה, ובאם אמה, ובאם אביה, ובבתה, ובבת בתה, ובבת בנה, ובאחותה בזמן שהיא קיימת. והאחין מותרין.

Accordingly, he is forbidden to have sex with her mother, and with her mother’s mother, and with her father’s mother, and with her daughter, and with her daughter’s daughter, and with her son’s daughter, and with her sister while his yevama is still alive. However, the other brothers who did not perform ḥalitza are permitted to her relatives.

והיא אסורה באביו, ובאבי אביו, ובבנו, ובבן בנו, באחיו, ובבן אחיו.

And she is forbidden to have sex with his father, and with his father’s father, and with his son, and with his son’s son, and with his brother, and with his brother’s son.

מותר אדם בקרובת צרת חלוצתו. ואסור בצרת קרובת חלוצתו.

The Mishnah states an additional principle: A man is permitted to have sex with a relative of a rival wife of his ḥalutza, i.e., his yevama with whom he performed ḥalitza. Since he did not perform ḥalitza with her, she is not regarded as though she had actually been married to him. However, he is forbidden to have sex with a rival wife of a relative of his ḥalutza, i.e., in addition to being forbidden to the relatives of his ḥalutza, he is also forbidden to their rival wives.

גמ׳ איבעיא להו: גזרו שניות בחלוצה, או לא?

Talmud: A dilemma was raised before the rabbis: In addition to the Torah prohibition against having sex with one’s wife’s close relatives, the rabbis decreed that it is also prohibited to have sex with one’s wife’s secondary relatives, i.e., those who are less closely related and so are permitted by Torah law. Since the rabbis decreed that a ḥalutza is to be regarded as though she had actually been married to the one who performed ḥalitza, such that her close relatives are forbidden to him, did they also decree that the prohibition of secondary forbidden relationships should also apply in the case of a ḥalutza, or not?