A Girl who has been Suckled by One's Sister or Sister-in-law is Mahram
Issue No. 2113- A person becomes maḥram to a girl suckled fully by his sister or his brother’s wife [by the milk relating to the brother] and he cannot marry her. Similarly, if a child has been suckled by one’s niece (the daughter of one’s brother, or sister), or the granddaughter of one’s brother or sister, he becomes maḥram to that girl and cannot marry her.
