Is it permissible for a son to marry his sister's daughter if the father's wife breastfed the sister's daughter twice to satiety during the first two years of life, while the son was not breastfed by the father's wife?

If your sister's daughter breastfeeds from your father's wife five full feedings before the age of two, then your father's wife becomes her foster mother, and your father becomes her foster father. You and her mother become her foster siblings, and your son becomes her foster nephew. If the breastfeeding does not fulfill the conditions for prohibition (five full feedings before two years of age), then marriage is permissible. However, it is preferable to avoid it as a precaution due to a jurisprudential difference of opinion.