Is it permissible to marry the daughter of the woman who was breastfed by my mother, knowing that my eldest sister was the one who was breastfed with her at that time, and I had not yet been born?

Scholars have differed on the number of sucklings that establish prohibition, and the preponderant opinion is that prohibition is not established with less than five sucklings. Based on this, it is permissible for you to marry the daughter of this woman if you are certain that her mother did not suckle five times from your mother, while taking caution. As for her suckling with your sister before your birth, it holds no weight. If she suckled five times from your mother, she becomes your sister through suckling, and her daughter becomes your niece. What is prohibited by lineage is also prohibited by suckling, as Allah Almighty says: "Prohibited to you are your mothers, your daughters, your sisters, your paternal aunts, your maternal aunts, the daughters of your brother, and the daughters of your sister."