Praise be to Allah.
If a man has a sister through breastfeeding, who was breastfed by his mother, or was breastfed along with him by another woman, he may marry her sister, because the mahram relationship here is limited only to the one who was breastfed, and not her sisters.
But if he was breastfed by the mother of the girl, then he is a brother to the girl and to all her sisters, so it is not permissible for him to marry any of them.
And Allaah knows best.
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