Sanhedrin 58A

שהיתה הורתו שלא בקדושה, ולידתו בקדושה ⁠– יש לו שאר האם, ואין לו שאר האב.

who was not conceived in sanctity, as his parents were still non-Jews, but his birth was in sanctity, as his mother converted before his birth, he has maternal kinship, i.e., his relationship to his mother’s relatives is recognized, but he does not have paternal kinship.

הא כיצד? נשא אחותו מן האם ⁠– יוציא, מן האב ⁠– יקיים. אחות האב מן האם ⁠– יוציא, מן האב ⁠– יקיים.

How so? If he married his maternal half sister, who was born before him and converted, he must divorce her. Although by Torah law they are considered unrelated, as a convert is considered to be reborn and all his previous family relationships are disregarded according to halakha, the rabbis prohibited their marriage, lest he think that as a Jew it is permitted for him to engage in behaviors that were previously forbidden to him. If she is his paternal half sister, he may maintain her as his wife, as it is permitted for a non-Jew to marry his paternal half sister. If he married his father’s maternal half sister, he must divorce her. If he married his father’s paternal half sister, he may maintain her as his wife.

אחות האם מן האם ⁠– יוציא. אחות האם מן האב, רבי מאיר אומר: יוציא, וחכמים אומרים: יקיים. שהיה רבי מאיר אומר: כל ערוה שהיא משום שאר אם ⁠– יוציא, משום שאר האב ⁠– יקיים.

If she is his mother’s maternal half sister, he must divorce her. If she is his mother’s paternal half sister, R' Meir says he must divorce her, and the Rabbis say he may maintain her as his wife. As R' Meir would say: With regard to any forbidden relative who is forbidden due to maternal kinship, whether the woman is his paternal relative, e.g., his father’s maternal half sister, or his maternal relative, he must divorce her; whereas if she is forbidden due to paternal kinship, he may maintain her as his wife.