Sanhedrin 28B

גוילי. בעו מיניה: מהו שיעיד אדם באשת חורגו?

scrolls of parchment. They asked of him there: What is the halakha as to whether a man can testify with regard to the wife of his stepson?

בסורא אמרי: בעל כאשתו.

In Sura they say that Rav gave the following answer: A husband is considered like his wife. Since he is married to the son’s mother, the son’s wife is considered like his daughter-in-law.

בפומבדיתא אמרי: אשה כבעלה. דאמר רב הונא אמר רב נחמן: מניין שהאשה כבעלה? דכתיב: ״ערות אחי אביך לא תגלה אל אשתו לא תקרב דדתך היא״. והלא אשת דודו היא! מכלל דאשה כבעלה.

In Pumbedita they say a different version of Rav’s answer: A wife is considered like her husband. Therefore, the wife of his stepson is considered like his stepson, and he cannot testify about her. As Rav Huna says that Rav Naḥman says: From where is it derived that a wife is considered like her husband? As it is written: “You should not uncover the nakedness of your father’s brother; you should not approach his wife: She is your aunt” (Leviticus 18:14). Isn’t she the wife of his uncle, and not his aunt, i.e., his father’s sister? It can therefore be concluded, by inference, that a wife is considered like her husband.

ובעל אמו, הוא ובנו וחתנו. בנו ⁠– היינו אחיו?

§ The Mishnah teaches that among the relatives disqualified from testifying are his mother’s husband, he, i.e., the mother’s husband himself, and his son, and his son-in-law. The Talmud challenges: The son of his mother’s husband is the same as his brother, i.e., his half brother, and the Mishnah already stated that his brother is disqualified.

אמר רבי ירמיה: לא נצרכה אלא לאחי האח.

R' Yirmeya said: Mentioning this case is necessary only for the half brother of the half brother, i.e., the son of his stepfather from another woman. Although they are not biologically related, they are considered relatives.