2,188. Are All Potential Relatives Prohibited?

Hilchos Issurei Biah 15:28

A woman prohibited as a forbidden relation (in the previous halacha) refers to any woman whose father or brother was alive when the asufi’s mother conceived him, or any woman who was divorced or widowed, because it’s always possible that this woman is his father’s wife or his paternal uncle’s wife.

Hilchos Issurei Biah 15:29

We know that a shetuki and an asufi are not prohibited from marrying every woman who might be a forbidden relation as follows: a valid child whose mother was asked his status isn’t prohibited from marrying any woman who could conceivably be forbidden to him as a potential relative. Leviticus 19:29 states, “Do not defile your daughter through promiscuous activity,” about which the Sages comment regarding a father marrying his daughter and a brother marrying his sister. If the law prohibited anyone who doesn’t know his father’s identity from marrying any woman who might potentially be a forbidden relation, this situation could never occur and there would be no concern about incest. We therefore infer that we don’t prohibit forbidden relations; rather, they are considered relatives out of doubt unless we know for sure that she is prohibited as a forbidden relation. If we were to say this, then no orphan who doesn’t know his father could ever marry out of fear of marrying a forbidden relation.