Source: Fatawa Aman Puri by Sheikh Ghulam Mustafa Zahir Aman Puri
Question:
Is it permissible to marry the granddaughter of a woman whose milk was suckled?
Answer:
Marriage is not permissible, because both are foster uncle and niece.
❀ The command of Allah the Almighty 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 also includes foster sisters, therefore, marriage to a foster niece is not permissible, because the relationship that is prohibited by blood is also prohibited by suckling.
❀ Sayyida Aisha (may Allah be pleased with her) narrates that the Messenger of Allah (peace be upon him) said:
إن الرضاعة تحرم ما تحرم الولادة
"Suckling also makes unlawful those relationships that are made unlawful by birth (blood)."
Reference: (Sahih Bukhari: 2646, Sahih Muslim: 1444)
