Al-Hasan (may Allah have mercy on him) was asked about a man who, after marrying, divorced his wife before having intercourse with her: may his father marry that woman? Al-Hasan (may Allah have mercy on him) said: No. Allah says: "And the wives of your sons who are from your own loins." [Surah al-Nisa: 23]
The Shaykh said: He only said "from your own loins", and Allah knows best, so that the wives of adopted sons would not be included in it. It is like what Allah said to His Prophet (peace and blessings be upon him): "So when Zayd had fulfilled his need of her, We married her to you, so that there would be no difficulty for the believers concerning the wives of their adopted sons." [Surah al-Ahzab: 37] So the wife of a grandson, or of anyone further down, and the wife of a son through nursing are also included in the prohibition.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13913