What do the honorable scholars and esteemed Muftis say regarding the following issue: Zaid and Umar are foster (radaʿi) brothers. Can Zaid’s younger brother marry the daughter of Umar’s elder sister? And are Zaid’s younger siblings and Umar’s younger siblings also considered foster siblings among themselves?
If only Zaid nursed from Umar’s mother, and Zaid’s other siblings did not, then Umar and his siblings are considered Zaid’s foster (raḍāʿī) siblings. However, there is no foster relationship established between Zaid’s other siblings and Umar or his siblings. Therefore, marriage between them or between their children is permissible according to Shari‘ah.
كما في صحيح مسلم للنيسابوري: 3652 - حدثنا قتيبة بن سعيد حدثنا ليث ح وحدثنا محمد بن رمح أخبرنا الليث عن يزيد بن أبى حبيب عن عراك عن عروة عن عائشة أنها أخبرته أن عمها من الرضاعة - يسمى أفلح - استأذن عليها فحجبته فأخبرت رسول الله -صلى الله عليه وسلم- فقال لها « لا تحتجبى منه فإنه يحرم من الرضاعة ما يحرم من النسب ». (4/ 164)