Is it permissible for a man to marry a woman who was suckled by his sister (the man's sister)?
It is not permissible for a man to marry his sister's daughters, whether they are her biological daughters or her foster daughters through رضاع (nursing), because Allah Almighty says: "Forbidden to you for marriage are your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, and your sister’s daughters" (Quran 4:23). And due to the saying of the Prophet (peace be upon him) regarding Hamza's daughter: "She is not lawful for me. What is forbidden by lineage is also forbidden by fosterage. She is my sister's daughter through fosterage." The fosterage that makes marriage unlawful, according to Islamic law, is five or more distinct acts of suckling.
Summarized from the full answer at Ftawy · reviewed Sep 2, 2026
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