His statement, “their two daughters,” means the daughter of his wife’s sister and the daughter of the sister of the woman observing a waiting period because of him.
His statement, “their two paternal aunts and their two maternal aunts,” means that it is not permissible for him to be married simultaneously to the woman observing a waiting period because of him and her paternal or maternal aunt; likewise, he may not be married simultaneously to his wife and her paternal or maternal aunt. Had the author—may Allah have mercy on him—and other scholars cited the verse and the hadith, it would have been clearer, more explicit, and more illuminating. Allah the Exalted says:
“And [also forbidden is] to be married to two sisters at the same time.”
The Prophet ﷺ said: “A woman must not be married together with her paternal aunt, nor a woman together with her maternal aunt.” (1)
The rationale is that combining such close relatives in marriage generally leads to severing ties of kinship, for it is well known that co-wives are often marked by enmity, hatred, and rancour between them. Therefore, in order to avoid severing kinship ties, the Sacred Law has forbidden being married simultaneously to two sisters, to a woman and her paternal aunt, and to a woman and her maternal aunt.
Thus, the categories between whom simultaneous marriage is forbidden are three: two sisters; a paternal aunt and her niece; and a maternal aunt and her niece. This is clearer and more explicit than the author’s formulation; moreover, it provides both the ruling and its evidence.
A paternal cousin and the daughter of her paternal cousin may be married simultaneously, because all relationships other than these three are lawful. This too is clearer than the statement of some scholars: “It is forbidden to be married simultaneously to two women such that, if one of them were hypothetically male, he could not marry the other because of blood relation or fosterage, but not because of marital affinity.” Although this is a valid criterion, it is a criterion that makes matters complicated