Source: Fatawa Aman Puri by Sheikh Ghulam Mustafa Zaheer Aman Puri
Question:
What is the ruling on marrying the daughter of a foster sister?
Answer:
It is not permissible to marry the daughter of a foster sister, just as it is not permissible to marry the daughter of a biological sister, because the relationships that are prohibited by blood relations are also prohibited by fosterage.
❀ Sayyida Aisha (may Allah be pleased with her) reports that the Messenger of Allah (peace be upon him) said:
إن الرضاعة تحرم ما تحرم الولادة
"Fosterage also makes those relationships unlawful that are made unlawful by birth (blood relations)."
Reference: (Sahih Bukhari: 2646, Sahih Muslim: 1444)
❀ The command of Allah, the Exalted, is:
﴿وَبَنَاتُ الْأُخْتِ﴾
Reference: (An-Nisa: 23)
"And the daughters of sisters (are also made unlawful for you)."
Here, the word "أخت" is general, which includes foster sisters as well, therefore, it is not permissible to marry a foster niece, because the relationship that is prohibited by blood is also prohibited by fosterage.
