Your brother's foster father is not a mahram to you
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Question: My younger sister was breastfed by my older sister along with her son, and my son was also breastfed by my older sister. Is it permissible for my son's father—that is, my husband—to be a mahram to my younger and older sisters, so that they may uncover themselves before him? Please inform us, may Allah reward you with good.
Answer: It is not permissible for your younger and older sisters to uncover themselves before your husband on account of your son being breastfed by your older sister. Rather, the husband of the sister who breastfed your son from her milk becomes his foster father, and a mahram to this child's wife, because she is the wife of his foster son, provided that the breastfeeding consists of five feedings or more within the first two years. This is based on the saying of the Prophet (peace and blessings be upon him):Sahih Muslim 1445i It is agreed upon that it is authentic.
Answer: It is not permissible for your younger and older sisters to uncover themselves before your husband on account of your son being breastfed by your older sister. Rather, the husband of the sister who breastfed your son from her milk becomes his foster father, and a mahram to this child's wife, because she is the wife of his foster son, provided that the breastfeeding consists of five feedings or more within the first two years. This is based on the saying of the Prophet (peace and blessings be upon him):
on account of fosterage as one is Mahram on account of consanguinity
يحرم من الرضاعة ما يحرم من النسب
Source
binbaz.org.sa
The Arabic text is copied verbatim from the original source, without any edits.