A number of scholars regarded it as mustahabb for a man to marry a woman who is not related to him, and they gave a number of reasons for that:
1 – The child would have good characteristics, because he would take characteristics from his father’s side and his mother’s side.
2 – There is no guarantee that they will not separate which would lead to severing of the ties of kinship.
It says in al-Insaaf (8/16): It is mustahabb to choose a woman who is religiously committed and fertile, a virgin of good lineage who is not a relative. End quote.
It says in Mataalib Ooli al-Nuha (5/9): A non-relative, because her children will have better characteristics, and because there is no guarantee that they will not separate, which in the case of marriage to a relative would lead to severing of the ties of kinship, which we are enjoined to uphold. And it is said that non-relatives give children with better characteristics and female cousins have more patience. End quote.
Al-Nawawi said in al-Manhaaj: It is mustahabb to choose a religiously committed virgin who is of good lineage but is not a close relative.” Al-Jalaal al-Mahalli said in his Sharh: “Not a close relative” means a non-relative or a distant relative. The one who is distantly related is better than one who is not related at all. End quote from Sharh al-Mahalli ma’a Haashiyat Qalyoobi wa ‘Umayrah, 3/208.
You can see that there is no text concerning this matter, rather it is the ijtihaad of the fuqaha’ which they based on these interests, which differ from one person to another, and from one type of relationship to another. A man may decide to marry a relative so as to protect her and honour her family, or she may be religiously committed and of good character.
The basic principle is that marriage is permissible. The Prophet (peace and blessings of Allaah be upon him) married Zaynab bint Jahsh who was the daughter of his paternal aunt, and he gave his daughter Zaynab in marriage to Abu’l-‘Aas who was the son of her maternal aunt, and ‘Ali married Faatimah, and he was the son of her father’s paternal uncle.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said, after quoting the reasons that the fuqaha’ gave, of seeking good characteristics in the children and the fear of severing the ties of kinship:
What they said is true, but if there is someone among the relatives who is better in terms of other considerations (such as religious commitment, lineage and beauty), then that is better. In the event that they are equal in these terms, then a non-relative is better.
So if a female cousin is religiously committed and of good character, and he is in a weak position and needs kindness and support, then undoubtedly such a marriage serves a great interest. A man should pay attention to his interests in this case. There is no shar’i text concerning this matter that has to be followed, hence a person should do what he thinks is in his best interests. End quote from al-Sharh al-Mumti’, 5/123.
The scholars of the Standing Committee were asked about marrying relatives and whether that causes retardation in the children.
They replied: There are no saheeh ahaadeeth which forbid marriage to relatives. The incidence of retardation happens by the will and decree of Allaah and is not caused by marriage to relatives as is widely believed. End quote.
Fataawa al-Lajnah al-Daa’imah, 18/13
And Allaah knows best.