[Is It Valid for a Man to Marry a Girl Who Was Breastfed by the Daughter of a Wife He Has Divorced?]

Answer—and Allah grants success:
As for the first question: It is not valid and not permissible for a man to marry a woman who was breastfed by his wife’s daughter. The proof is His saying Most High: “And your stepdaughters who are under your guardianship [born] of your wives unto whom you have gone in—but if you have not gone in unto them, there is no blame upon you” [Al-Nisāʾ:23]. Thus Allah the Exalted forbade the stepdaughter—that is, the daughter of the woman—if he has consummated with her mother; and the daughter of the stepdaughter has the same ruling, downward through her line. And breastfeeding carries the ruling of lineage; so the daughter through breastfeeding is like the daughter through lineage in this chapter.
As for the answer to the second and the third questions: There is no objection to what is mentioned therein. It is valid for a man and his son to marry a woman and her daughter: the younger to the younger and the elder to the elder—or the reverse.
The proof is His saying, Exalted is He, after listing the prohibited degrees in Sūrat al-Nisāʾ: “And lawful to you are [all others] beyond these.” [Al-Nisāʾ:24]
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1