Is it permissible for me to marry the sister of a girl with whom I was breastfed by the wife of my paternal uncle, knowing that her mother does not know if she breastfed me five full feedings?

If you were suckled by a woman with a milk-kinship-establishing suckling (five full sucklings during the first two years of life), she becomes your mother and her children become your milk-siblings. Therefore, it is not permissible for you to marry her daughters. However, if five full sucklings did not occur, then the prohibition is not established, because milk-kinship is not established by doubt. It is preferable and more cautious to avoid marrying that girl, out of consideration for the opinion of the jurists who hold that prohibition is established by any amount of suckling.