Source: Fatāwā Amunpuri by Shaykh Ghulām Mustafa Zaheer Amunpuri
❖ Question:
What is the ruling on marrying a girl (raḍīʿah) who was breastfed by a woman, to the son of that same woman?
✿ Answer:
Such a marriage is not permissible, because the two are considered foster siblings.
Once a woman breastfeeds a child, all her biological children become forbidden in marriage to the child who was breastfed. Therefore, the boy is a foster brother to the girl.
❀ Allah ﷻ says:
﴿وَأَخَوَاتُكُمْ﴾
“And your sisters are forbidden to you (in marriage).”
📚 Surah al-Nisā’ (4): 23
This includes both biological and foster sisters (akhawātukum includes raḍāʿī sisters as well).
✅ Conclusion:
① A girl who is breastfed by a woman becomes a foster daughter to that woman.
② Her children—including sons—become foster siblings to the girl.
③ Therefore, marriage between a foster brother and sister is absolutely forbidden (ḥarām).
