We have reviewed request no. 866 for the year 2005 which includes the following question:
My sister wants to marry my cousin (the son of my paternal uncle). My brother was nursed with him, and we do not know how many nursings occurred: whether it was a more or less than five. Is it permissible for her to marry him?

 

If you mean that your brother and your cousin had were breast-fed by your paternal uncle’s wife or any woman other than your mother, then it is permissible for your sister to marry your cousin regardless of the number of nursings.
However, if you mean that your cousin was breast-fed by your mother, then it is unlawful for your cousin to marry your sister, on the condition that five satiating nursings have occurred within the first two years of the child’s life. This is because he would then be the brother-through-nursing of all your mother's children, including the sister who wants to marry him.
According to the Shafi‘is and Hanbalis, if it is not known whether five nursings had occurred or not, it is then permissible to marry, because they maintain that the prohibition is not established until five satisfying nursings occur. As long as there is doubt whether five nursings have taken place [it remains lawful] since what is certain (i.e. marriage) is not negated by doubt (i.e. the occurrence of five nursings). This is the opinion relied upon for fatwa because the matter so commonly occurs.

God the Almighty knows best.