Does the paternal aunt's daughter become a foster sister to the questioner and his older brother, bearing in mind that their mother breastfed the paternal aunt's daughter along with their younger sister, while the questioner and his brother were nursed with their mother's first husband's milk?
If your maternal aunt's daughter was nursed by your mother five full breastfeedings, then she becomes your sister through رضاعة (nursing), and what is prohibited by lineage is also prohibited by nursing. Therefore, she is one of your mahārim (unmarriageable kin).
Summarized from the full answer at Ftawy · reviewed Sep 2, 2026
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