In the Shari`ah, when a ghayr mahram woman breastfeeds a child less than 2 years of age, she becomes the “milk-mother” of the child. She is now a mahram to the child (essentially, like his actual mother) and as a result, she cannot marry him, nor is she obligated to observe purdah (hijaab/niqaab) in front of him when he grows older. Consequently, if the woman has a husband, he become the milk-father of the child, while the woman’s sisters will become his milk-aunts. If the woman has children, they will become the child’s milk-siblings. Essentially, suckling a child will result in a whole chain of new relationships to form. The only condition is that the child is suckled before he reaches the age of 2 Islamic years.
In the Shari`ah, this is known as radhaa`ah, or relations through suckling. Although radhaa`ah affects laws of marriage and purdah, it has no affect on the laws of inheritance. Thus, there are no ties of inheritance between milk-relatives. In the scenario described above, the child may not inherit from his milk-mother or any of his other milk-relatives, and nor can they inherit from him.