Is it permissible for a man to marry the granddaughter of his stepdaughter (the daughter of his stepmother's daughter) if his stepmother breastfed this daughter?

The issue is based on the ruling concerning marriage to the daughter of one's stepmother. If the stepmother's daughter is a foster sister, then the man is a maternal uncle to her daughter through fosterage. If she is a stranger to him, then her daughter is also a stranger. Scholars have differed on the ruling regarding the fosterage of the stepmother's daughter:

The Malikis hold that the girl becomes forbidden if her fosterage continues after her mother's marriage, making her a foster sister to the children of her mother's husband. The Hanafis and Shafi'is (according to the more correct opinion) hold that the milk belongs to her first husband unless she gives birth to a child from her second husband. Thus, she does not become a foster sister unless she was suckled after her mother gave birth to their father's child. The Hanbalis added that if the milk increases due to pregnancy from the second husband, then the milk belongs to both of them.

Therefore, if your father's stepdaughter continued to be nursed after your father married her mother, we advise you to refrain from marrying her daughter as a precautionary measure and to avoid disagreement.