“Islamic law has placed certain restrictions on the choice of your spouse depending upon blood relationships and religious affiliations.”
(a) Restrictions based on Relationship
There are certain blood relations which are considered haraam for you as far as marriage is concerned. (As a general rule, anyone who is your mahram is forbidden to you for marriage.) The list of such relatives is given in the Qur’an as follows:
For Man: mother, daughter, paternal aunt, maternal aunt, niece, foster-mother, foster-sister, mother-in-law, step-daughter, daughter-in-law, all married women, sister-in-law (as a 2nd wife) (See the Qur’an, ch. 4, verse 23-24)
For Woman: father, son, paternal uncle, maternal uncle, nephew, foster-mother’s husband, foster-brother, father-in-law, stepson, son-in-law.
(b) Restrictions based on Religion
A Muslim man can marry: a Muslim woman and a non- Muslim woman. However, if there is danger of being misled, then it is haraam.
He can also marry a Jewish or Christian woman in mut’a only. But he cannot marry a woman of any other faith.
A Muslim woman can marry: a Muslim man or a non- Muslim man, although it is better not to do so; and if there is danger of being misled, then it is haraam. But she cannot marry a non-Muslim man.
(c) Cousin Marriages
Though Shariah does not forbid marriage between first cousins, but there are opinions advocating against them mainly due to a probable risk of the offspring inheriting genetic defects/diseases.