The categories forbidden through blood relationship may be summarized as follows:

First: ascendants, however far removed.

Second: descendants, however remote.

Third: the descendants of one’s nearest ascendant, however remote. Thus, the descendants of one’s father are one’s brother and sister, and the same applies to one’s mother.

Fourth: the descendants of more remote ascendants. We do not say, “however remote,” meaning: the daughters of one’s grandfather and the daughters of one’s grandmother, but not their daughters.

These are four governing principles. If they become unclear to you, return to the clear matter: the noble verse.

The author mentions the third category of those permanently forbidden in marriage in his statement:

“The woman involved in mutual imprecation is forbidden to the one who engaged in mutual imprecation with her, …”

“The woman involved in mutual imprecation is forbidden to the one who engaged in mutual imprecation with her.” A woman involved in mutual imprecation is one whom her husband has accused of adultery, while she has neither admitted it nor has he produced evidence for his accusation. If, in this situation, she demands that the prescribed punishment for slander be carried out against him, he may avert it through mutual imprecation. The judge brings them both forward and says to him: “Testify four times concerning your wife, and on the fifth time say that Allah’s curse be upon you if you are lying.” He then swears four times, and in the fifth testimony says, “Allah’s curse be upon him if he is lying.” The judge then says to her: “Swear that he is lying.” She swears by Allah four times that he is indeed lying, and on the fifth says, “Allah’s wrath be upon her if he is telling the truth.” Once this has been completed, the judge separates them permanently; she is never lawful for him again.