Which relatives are forbidden to marry?
There are 3 types of kinship that makes the marriage illegal:
a- Blood Kinship: A man cannot marry his following close relatives: mother, grandmother, own daughter, own granddaughter, sister, daughter/granddaughter of sister, daughter/grandaughter of brother, father’s sister and mother’s sister.
b- Relatives By Marriage: A man is eternally forbidden from marrying the mother and grandmother of the woman he married. It is the same for the sister and granddaughter of his wife with whom he has had sexual intercourse. The wives of his father, grandfather, son and grandson are also eternally forbidden to him. These relations and prohibitions remain, although the wife dies or they get divorced.
c- Milk Kinship: The woman who nursed a person until two years old and her husband and her children and other children she nursed become relatives with that person through nursing. Although there are some exceptions, a person is forbidden from marrying the women he is related through nursing just as the ones he is forbidden through blood kinship. For example a person cannot marry his milkmother, sister of milkmother, sister of milkmother’s husband and milksister.
Relatives apart from the ones above are not forbidden to marry. Related verses are as follows:
“And do not marry those [women] whom your fathers married, except what has already occurred. Indeed, it is an outrageous act and an evil way.
Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your fathers’ sisters, your mothers’ sisters, your brothers’ daughters, your sisters’ daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives’ mothers, and your step-daughters under your guardianship [born] of your wives with whom you have had sexual intercourse. But if you have not had sexual intercourse with them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is The Forgiver and The Merciful” (An-Nisa/ The Women 4:22-23)