Is it permissible for my cousin to marry my sister if my maternal grandmother breastfed him, and what if there is doubt about that?

If your grandmother breastfed your cousin five times within the first two years of his life, he becomes her son and your mother's brother through rada'ah ( দুধ সম্পর্ক ), making it impermissible for him to marry your sister. This is because she would be his sister's daughter through rada'ah, and what is forbidden by lineage is also forbidden by rada'ah.

However, if his breastfeeding by her is not established, the default is that she is not forbidden to him. If breastfeeding is established, but there is doubt about the number of feedings, then, out of caution, marriage to her should be avoided.