(2) his wife’s milk-daughter, provided the nursing was with milk other than his own;
(3) and the wife of his milk-son;
(4) and the wife of his milk-father who did not herself nurse him.

These women would be prohibited only by affinity, not by blood; there is no lineage connecting him to any of them. Al-Qurṭubī, however, comments in his Tafsīr: “The wife of one’s milk-son is forbidden, even though the son is not born of one’s loins, by unanimous agreement based on the Prophet’s ﷺ statement, ‘Whatever is unlawful through breastfeeding is unlawful through lineage.’”

Ibn Kathīr writes: “If it is asked, ‘Why is the wife of one’s milk-son forbidden—as the majority hold, with some even claiming consensus—when the son is not one’s natural offspring?’ the correct answer is found in the Prophet’s ﷺ words, ‘Whatever is unlawful through breastfeeding is unlawful through lineage.’”

Shaykh al-Shinqīṭī states in Aḍwāʾ al-Bayān: “The prohibition of the woman married to one’s milk-son is taken directly from the explicit statement of the Prophet ﷺ that ‘Whatever is unlawful through breastfeeding is unlawful through lineage.’ The women made unlawful through affinity are four:

1. a father’s wife, however far up the line;
2. a son’s wife, however far down;
3. the mothers of one’s wife, however high they ascend;
4. her daughters, however low they descend.

All of these become forbidden by the mere marriage contract, except the step-daughter (al-rabība), who is not forbidden until consummation with her mother has taken place.

Fifth point. Shaykh ʿAbd Allāh Ābā Buṭayn said: Contracting marriage with a woman while her sister—or any comparable relative—is still in her waiting period is like marrying a fifth wife while the fourth is still in her ʿidda. If the divorce involved is revocable, the contract is invalid by unanimous agreement; if the ʿidda is from an irrevocable divorce, there is a well-known difference of opinion, but the authoritative position is that it is prohibited.