The Ruling on Giving the Daughter in Marriage to the Father's Maternal Uncle and the Ruling on Marrying the Paternal Uncle's Daughter by One Who Was Breast-fed With Her Once

Question :

Is it permissible for a man to give his daughter in marriage to the maternal uncle of his father, and is it permissible for him to marry the daughter of his paternal uncle, who was breast-fed for one day or a part of it with him?

Answer:

Regarding the first question, it is not permissible for the questioner to give his daughter in marriage to his father's maternal uncle, because his father's maternal uncle is the maternal uncle to him and his descendants - both his offspring and the succeeding generations, based upon the generality of Allah, Most High's Words:
"your sister's daughters.

So the uncle may not marry the sister's daughters, or even their descendants.

As for the second question, if the aforementioned breast-feeding took place five times or more, and it was during the first two years, then this breast-feeding constitutes forbiddance. And it is not permissible for the questioner to marry his paternal uncle's cousin who was breast-fed with him or with one of his brothers. However, if it was less than five times, or it was after the first two years, then there is no effect from it. And what is considered by the Islamic Law to be fostering is that the child sucks the milk from the breast, and if he leaves it, that is considered one feed. Then if he returns to it, that is considered a second feed and so on, until he completes five. In light of this, it is clear that what constitutes fostering is what we have mentioned, not that it was a day or part of a day, because the child might complete what is considered to be fostering by the Islamic Law within less than an hour, or the fostering which extends prohibition (of marriage) might not be completed in five days or more. And may peace and blessings be upon our Prophet, Muhammad and upon his family and Companions.

Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 5 Pages 231-232