Professor `Abdul-Kareem Zeidaan, former professor of Islamic Jurisprudence at Baghdad University, states the following: “Concerning what is preferable in the choice of a wife, Shafi`is hold the view that the wife should not be of a close relation. Supporting their view, they cite the Hadith that reads: “Do not marry a close relative lest your offspring be emaciated.” But they quoted Ibn Salah as saying that this Hadith is not mentioned in any of the authentic books of Hadith. Moreover, As-Subki said that this opinion cannot be accepted, for lack of textual evidence, and also because of the fact that the Prophet, peace and blessings be upon him, married his cousin `Ali to his daughter Fatimah in spite of being close relatives.
In “Mughni Al-Muhtaaj” on the Shafi`i school of Fiqh it is stated: “Ash-Shafi`i stated that it is preferable for a man not to marry from amongst his folk. Az-Zingaani 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.”
Az-Zingaani further states: “ Extending the scope of social relations 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”, the Hanbali jurist Ibn Qudamah 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 lest 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 couple. If the couple 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.
What Is Better Then?

The most correct view is to consider each issue separately, because such a choice that is based on religion is to 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. The rationale here should be “Charity begins at home.” 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 issue should be weighed according to the religious interest that should be given priority.

Almighty Allah knows best.