There is a proverb in Persian which says, “A girl’s marriage with her cousin is made in heaven”. Is it really as such? The answer is that: Who knows about the heaven? Obviously no one knows about it except God, the Exalted, and the infallible leaders? No matter how much you search, you will find but very few traces of this crucial phrase in Muslims’ hadith books.
In fact, a tradition opposed to the aforementioned proverb has been narrated from the Messenger of Allah (pbuh) who said: “Marry with a stranger so as not to bear (give birth to) a weak child.”
Imam Sadiq is reported as having said: “Do not marry with close relatives because a weak child will be born.”
Perhaps, it is because of these traditions and doctors’ preventive pieces of advice and those of the genetic experts that some contemporary jurisprudents have considered family marriage as “abominable”.
In any case, it should be noted that, as per the Islamic law, such a marriage is not forbidden.
 - In this regard, you can search Shia’s and Sunnis’ extensive fiqh and hadith sources in the following source: “Ahlulbayt Libraries” produced by Jurisprudential Encyclopedia (Markaz Mu’jam Fiqhi) affiliated to the office of Grand Ayatollah Sayed Ali Sistani in Qom.
 - Al-Nehayah, Ibn-e Athir, Vol.3, pg 106; Al-Mujazat Al-Nabawiyah, Sharif Razi, pg.92; Lesaan Al-Arab, Vol.14, pg. 489.
 - Lesaan Al-Arab, Vol.1, pg, 639; Masalekul Ifhaam, Shahid Thani, Vol.2, pg.38; Al-Mahajjatul Baydha, Fayz Kashani, Vol.3, pg.94.
 - For instance, see: Health and Family Planning, Tehran: Department of Public Health and Prevention (Health Ministry), 1373, pg.121 (Grand Ayatollah Naser Makarem Shirazi’s Answer to a Question by Elam’s Faculty of Medicine).