There is no Objection to Marrying Your Brother's Foster Sister
Question :
Is it permissible for me to marry a young woman whose elder sister is my younger brother's foster sister? May Allah reward you with the best of rewards.
Answer:
There is no objection to marrying your brother's foster sister, if it was your brother who was breast-fed by her mother and you were not breast-fed (by her). Likewise, if the one who breast-fed, the sister of the young woman was breast- fed by your mother, then the young woman is permitted to you and her sister's having been breast-fed by your mother would not harm you, nor the breast-feeding of your brother by her mother. And Allah knows best.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 5 Page 229
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