Although cousin marriage is allowed in Islam, there are some weak narrations (extremely weak) discouraging it..
“Marry outside the family, lest the offspring be thin and weak.” [Ibn Hajar, Talkhis al-Habir]
But Hazrat Umar has suggested to marry outside of the family/ tribe, when he saw the people of that tribe become very weak..
“You all have become thin and weak, so marry outside the family.” [Iraqi, Takhrij Ahadith al-Ihya’]
Hanbali fiqh explicitly mentions that it is recommended (mustahabb) for a man to marry outside the family, as doing so normally leads to healthier and stronger offspring.