2,034. A Wife's Sister

Hilchos Issurei Biah 2:8

If a man has relations with one of the six perpetually-forbidden women after his wife’s death, they are liable to kareis (spiritual excision) but they are not executed by the courts. This is derived from Leviticus 20:14, “Both he and they shall be burned in fire,” which applies only when both his wife and the woman with whom he had relations are alive. If both women are not alive, there is no execution by burning.

Hilchos Issurei Biah 2:9

Similarly, the sister of one’s wife is a prohibited relationship to him until his wife dies. This applies to both a maternal and a paternal sister, whether conceived in or out of wedlock.