Your Brother's Father Through Breast-feeding is Not a Mahram For You
Question :
My youngest sister was breast-fed by my eldest sister, along with her son, and my son was also breast-fed by my eldest sister. Is it permissible for my son's father-that is, my husband to be a Mahram to my youngest and eldest sisters, so that they may unveil themselves before him? May Allah reward you with goodness.
Answer:
It is not permissible for your youngest and eldest sisters to unveil themselves before your husband based on your son being breast-fed by your eldest sister. Only the husband of your sister who breast-fed your son with her milk will be a father through breast-feeding and forbidden to the wife of this son. This is because she is the wife of his son through breast-feeding and this is based upon the condition that the breast-feeding took place five times or more during the first two years, according to the saying of the Prophet ﷺ:
"That which is forbidden due to kinship is forbidden due to breast- feeding."
Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Pages 118-119