They Are Maternal Uncles to You Through Breast-feeding
Question :
My mother was breast-fed by another woman, and this breast-feeding woman has co-wives; are the daughters of these co-wives considered sisters to me as well, or not?
Answer:
This breast-feeding woman is considered your grand- mother, since she suckled your mother. Her husband is considered your mother's father and your grandfather from your mother. Her co-wives are considered the wives of your grand- father from your mother, and their children are your maternal uncles and your mother's brothers, since their father is your grandfather. They are the children of your grandfather, so they will be maternal uncles to you through breast-feeding.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 6 Pages 143-144