Are the children of the woman who breastfed me considered my siblings, and what is the ruling on greeting them? And will I be an aunt to their children? And is it permissible for my children to marry them, bearing in mind that she breastfed my male cousin and another girl?

Whoever breastfeeds a child five times or more within two years, that child becomes a sibling to all her children and to those she has breastfed. What is prohibited by lineage is also prohibited by breastfeeding. So, if you were breastfed by a woman five times within two years, all her children and those who were breastfed with them through valid suckling are considered your siblings. Their children are your mahram, and your children are their maternal uncles or aunts.