2,171. Marrying Paternal Relatives

Hilchos Issurei Biah 14:13

If a non-Jew was married to his mother or sister and they converted, they are made to separate as was discussed in the previous halacha. If he was married to any of the other prohibited relationships and they both converted, they are not compelled to separate. Under rabbinic law, a convert may not marry relatives on his mother’s side after they convert but he may marry relatives on his father’s side. This is true even if he knows them to be his relatives, such as in the case of twins in which it’s obvious that they have the same father. Nevertheless, the Sages didn’t enact against a convert marrying relatives on his father’s side. Therefore, a convert may marry the wife of his brother from the same father, his father’s brother’s wife, his father’s wife and his son’s wife. This is so even if they married his brother, father, father’s brother or son after conversion. The same is true of his mother’s sister from the same father, his own sister from the same father and his daughter who converted. He may not marry his sister from the same mother, his mother’s sister from the same mother or a woman who was married to his brother from the same mother after he converted. If a woman was married to his brother before he converted, she is permitted to him.

Hilchos Issurei Biah 14:14

If twin brothers were conceived before their mother converted and born after, each of them would be liable for having relations with his brother’s wife.