Dr. `Abdul Kareem Zeidaan, former professor of Islamic Jurisprudence at Baghdad University, states the following: “Concerning what is preferable in the choice of a wife, scholars of the Shafi`i School of Fiqh are of the opinion that the wife should not be of a close relation, and justified this by a hadith ascribed to the Prophet (peace and blessings be upon him): “Do not marry a close relation or else your offspring be emaciated [i.e. weak.]” But they quoted Ibn As-Salah as saying that this hadith is not mentioned in any of the authentic books of Hadith. Moreover, Al-Subki said that this opinion should not be valid, for lack of textual evidence to support it, and also because of the fact that the Prophet (peace and blessings be upon him) gave Fatimah (the Prophet’s daughter) in marriage to `Ali in spite of their being closely related.
In Mughni al-Muhtaj, a Shafi`i book of Fiqh, it is stated: “Ash-Shafi`i stated that it is preferable that a man does not marry from his folk. Az-Zinjani attributed this to the fact that one of the objectives of marriage is to unite people for the purpose of achieving solidarity and mutual cooperation.”
I think that the acceptable reason for recommending marrying someone outside the family circle rather than a close relative is that which Az-Zinjani has stated: that is extending the scope of social relations which leads to strengthening relations and enhancing cooperation among people. Therefore, recommending men to marry from outside the family is not because of some fault in close relatives. People usually say: marry someone outside the family circle in order to give birth to healthy children.
In his book Al-Mughni, Ibn Qudamah – the Hanbali jurist – says: “A man should choose a wife from outside the family circle because she would most probably give birth to a more intelligent child than the one his kindred would most likely have. That is why people recommend such a marriage and say: Do not marry someone of your kindred or else your offspring will be emaciated, which means: you should marry someone from outside the family circle in order to give birth to healthy children. Moreover, disagreements leading to divorce may occur between the spouses; in such case, if the spouses are relatives their divorce may lead to severing the ties of kinship which Allah has ordered us to maintain.” This opinion could help demonstrate the rationale behind the preference of marriage to someone from outside the family circle.
The preferred opinion concerning whether to choose a relative or someone from outside the family circle is to consider each case separately, because such a choice is meant to realize a religious interest that should be given priority whether it is realized through marrying a relative or someone from outside the family circle. This of course differs according to circumstances; for instance, if a man’s relative is an orphan who has no one to support her, and is a religious girl, marrying her will certainly be far more preferable than marrying someone from outside the family circle, because relatives are more deserving to kindness and care than any other one. Marrying this orphan is undoubtedly a kind act that realizes a religious interest which should be given precedence. Moreover, if a girl has many relatives who would like to marry her, the right thing is that some of them should give her up and marry someone from outside the family. In addition to this, if marriage to someone from outside the family circle would most likely strengthen the ties between two families and remove old enmity, such a marriage is preferable in this case. Therefore, every case should be weighed according to the religious interest that should be given priority.