has the other’s relatives as in-laws. A man’s relatives are his wife’s ahmā’ (in-laws through her husband), and a woman’s relatives are her husband’s akhtān (in-laws through his wife). These four categories become forbidden in marriage upon conclusion of the marriage contract, except for the stepdaughter: she does not become forbidden until the marriage to her mother has been consummated. Allah stipulated this condition only for the stepdaughter, leaving the prohibition concerning the others unrestricted.

For this reason, the Companions said: “Leave unrestricted what Allah has left unrestricted.” This is the position of the four imams and the overwhelming majority of scholars. As for the daughters and mothers of these two—the father’s wife and the son’s wife—they are not forbidden. A man may therefore marry the daughter of his father’s wife or his son’s wife, by scholarly consensus, since she is not herself one of the wives of fathers or sons. The word halīla means “wife,” and a wife’s daughter and mother are not themselves wives. The stepdaughter is different, since a stepchild’s child is also a stepchild, just as one’s child’s child is also one’s child. Likewise, the mother of one’s wife’s mother is also a mother of one’s wife. A daughter of one’s wife’s mother, however, is not forbidden, since she is not a mother.

For this reason, some jurists have said: “The daughters of women forbidden in marriage are also forbidden, except the daughters of paternal aunts, maternal aunts, wives’ mothers, and fathers’ and sons’ wives.” Thus, they regarded a stepdaughter’s daughter as forbidden, unlike the daughters of the other three categories. I know of no disagreement concerning this.

If a man has intercourse with a woman under what he believes to be a marriage, paternity is attributed to him and the marriage prohibitions arising from affinity are established, by scholarly consensus as far as I know—even if that marriage is invalid before Allah and His Messenger. An example is a disbeliever who contracts a marriage forbidden in Islam: paternity is attributed to him through it, and affinity is established. Each spouse is therefore forbidden to marry the other’s ascendants and descendants, by scholarly consensus. The same applies to any intercourse that a person believes is not unlawful, although it is in fact unlawful, such as