Are my sisters' wet-nurses' sisters considered my aunts and, therefore, unmarriageable to me?

The brothers and sisters of the nursing mother are maternal uncles and aunts to the nursling alone, and not to his siblings. There is no fosterage kinship between the nursling's siblings and his parents by lineage, and the nursing mother's family; they are strangers to each other. Based on this, your foster sister's sisters are not your aunts, unless you were nursed by their sister.