I am told by a Muslim scholar from a traditional-tribal society that the cousin has the social right upon his female cousin and that she is not to be offered to him first. No one may propose to her until he has expressed his wish not to marry her. In a way, within the Arab, particularly tribal societies, they consider marrying within the family, more honourable, more protective; keeping lineage pure and well established.
However, there is a statement which is attributed mistakenly to the Messenger of Allah, "Marry from outside the family, otherwise your offspring will be weak." In fact this, or something similar, is correctly attributed to Umar ibn Al-Khattab saying to the family of As-Saib, "Your offspring are becoming so thin and weak. Marry outside your close of kin." In discouraging this marriage, Al-Ghazali in his Ihya Ulum ad-Deen says, "Familiarity and close family tie weaken the sexual desire in both of them. As a result, children become weak." This is not a good reason. For surely, when partners marry, after a few months they become familiar, there may be nothing new to attract as they know each other inside out, but the natural desire is there.
However, research nowadays is showing that the marriage of close relatives leads to the accumulation of negative inherited qualities. For scientific reasons therefore it may be advisable to marry from afar.