What is the ruling regarding the daughters of my sister's husband from another wife whom he divorced after marrying my sister, knowing that my sister breastfed the youngest of that divorced wife's sons? And are they considered mahrams to me?

What is forbidden by lineage is also forbidden by breastfeeding. Thus, the suckled child is considered a son to the wet nurse and the owner of the milk. This applies exclusively to the suckled child himself, not to his siblings. Therefore, the daughters of your sister's husband from a previous wife are not considered your mahram, unless there was lawful breastfeeding between you. Your sister's breastfeeding of her youngest son is specific to him and does not extend to his sisters. It is a condition for breastfeeding to establish prohibition that there be five distinct sucklings.