In reality, breastfeeding (raḍāʿah) establishes the same prohibitions as blood relations by birth, as the Messenger of Allah ﷺ said:
«يحرم من الرضاعة ما يحرم من الولادة»
“Breastfeeding makes unlawful what blood relations make unlawful.”
(Ṣaḥīḥ al-Bukhārī, 2644)
According to this ḥadīth:
- The one who drank his sister’s milk becomes her foster-son (ibn raḍāʿī).
- The sister’s children become his foster brothers and sisters.
- His children will be the nephews and nieces (raḍāʿī) of the sister’s children.
Therefore:
Marriage between them will not be permissible, as they are considered foster relatives under Sharīʿah.
