The present verses are an extension of the description of evil practices common in Jahiliyyah which have been taken up earlier. A detailed discussion appears in the Commentary.
Commentary:
These verses give details of محرَّمات muharramat, that is, women with whom marriage is prohibited. Some of them are prohibited forever, never becoming lawful under any condition. Some are not permanently prohibited. They become lawful under some particular circumstances.
There are three kinds of permanently-prohibited women referred to in the first category. They are those by kinship, by fosterage and by relationship through marriage, and are permanently prohibited. The kind mentioned at the end remains prohibited until they are bound in marriage with someone else.
An explanation of the verses follows with relevant part of the verse appearing in the lead: وَلَا تَنكِحُوا مَا نَكَحَ آبَاؤُكُم (And do not marry those of women whom your fathers had married) (verse 22): During the days of Jahiliyyah, people had no qualms when they went ahead and married the wife of their father after his death. In this verse, Allah Almighty has prohibited this shameful practice; an evil which invites His wrath. How can someone keep calling a woman his mother for a long time and yet stoop so low as to make her his wife after the death of the father?
Ruling:
Marrying the legally wedded wife of the father has been declared unlawful in this noble verse. There is no restriction here to indicate marital consummation by the father. So, the fact that the father has married a woman is enough to forbid the son from entering into marriage with that woman. It will never be lawful. Similarly, it is not correct for the father to marry the wife of his son, even if the marriage of the son is limited to formal 'nikah' and his wife have not yet come to live with him. (Ibn ` Abidin)