All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The verse which you referred to is evidence that the wife of the son is forbidden for his father, and this is explicitly stated, whether the wife is still in a marriage bond with the son or he has divorced her, and this is whether or not he has children from her. So, in principle what is absolute and general should be understood as such until there is a text which confines it. This is what the scholars understood from that verse, and so, they forbade the wife of the son for his father. So, it is the one who says that the verse applies to a particular case who is required to provide evidence.
The wife of the son is eternally forbidden for his father whether or not the son had children from her. So, where is the evidence to say that the wife of the son who has no children from him is not forbidden to his father? The existence of the children or their non-existence does not affect the ruling. The reason why it is wrong for the father to marry his son’s wife is that he contradicted the ruling of Allaah and married someone who is eternally forbidden for him. Marrying the Mahram women, whether it is the wives of the sons or else, is a grave matter, and it is a void marriage according to the consensus of the scholars, so one should be careful.
Dr. Wahbah Az-Zuhayli said: “The wife of his male descendants. . . whether his descendant consummated the marriage with her or not even after he separated with her by divorce or death, like the wife of the son or grandson or great grandson as Allaah says (what means): {And [also prohibited are] the wives of your sons who are from your [own] loins.}[Quran 4:23]
The marriage contract with her would be void and it does not have any consequences. The scholars said: “The prohibition is confirmed with the same contract regarding the wife of the father and the wife of the son.”
The Hanafi School also prohibited - along with the wife of the father or the son - the woman with whom they had sexual intercourse by Zina (adultery) or by a defective marriage contract, because just having sexual intercourse (even once) is enough for them for that woman to be prohibited for the man (the father or the son).” [Al-Fiqh al-Islaami wa-Adillatuh]