As for its being based on what generally occurs, that is because—especially in the early days of Islam—a woman’s daughter would generally live with her when she married.
As for the second point, namely that it indicates the underlying reason, it is as though Allah were saying: She is forbidden to the husband because she is like one of his own daughters. She is under his care, and he looks upon her as one who raises and nurtures her. Thus, one may find that she is seven, ten, or twelve years old, coming to her mother’s husband, serving him food, and uncovering her face before him, entirely as though she were his daughter. It would therefore be inappropriate for him to enter upon her and marry her.
The view held by the majority of scholars is the stronger one: only one condition is required for a stepdaughter to become forbidden to her mother’s husband, namely, that he has consummated the marriage with her mother.
Thus, if he contracts marriage with a woman and divorces her before consummation, or if she dies before consummation, then her daughters are lawful for him to marry, even if they are under his care. This is the view of the majority, and it is the stronger view.
These four categories become forbidden through marriage. But do they also become forbidden where the relationship is established through breastfeeding? In other words, are a woman’s husband’s foster-father and foster-son forbidden to her, and are a man’s wife’s foster-daughter and foster-mother forbidden to him? For example, a man marries a woman who has both a foster-mother and a biological mother. Her biological mother is forbidden to him; but is her foster-mother likewise forbidden to him or not?
There is a difference of opinion on this issue. The majority of scholars, including the four imams, hold that the prohibition is established through marital ties. An alleged consensus on this has been reported, but it is unsound. They therefore maintain that a husband’s foster-father and foster-son are forbidden to his wife, and that a wife’s foster-mother and a daughter