2,035. Marrying One of the Seven Relatives

Hilchos Issurei Biah 2:10

If a man had relations with one of his wife’s seven aforementioned relatives, whether he did so intentionally or unintentionally, then even though he and the woman are both subject to execution or kareis (excision), he is not prohibited to have relations with his wife. The exception is the sister of the woman one has betrothed, in which case one’s his wife is rendered prohibited to him as was discussed in the laws of divorce.

Hilchos Issurei Biah 2:11

If a man has extramarital relations with a woman, her seven relatives are not rendered prohibited to him but the Sages prohibited one who had promiscuous relations from marrying the woman’s seven relatives during her lifetime. The reason for this is because the woman will invariably visit her relatives, causing the man to end up secluded with her. Since they have already been intimate, we are concerned for the possibility that they may engage in prohibited relations. Even if a man is only suspected of promiscuous relations with a woman, he may not marry one of her relatives until the woman with whom he is suspected dies. If he should marry the relative of a woman with whom he was suspected, he need not divorce her.