Fucking your cousin leads to poor outcomes.

"As of 2003, an average of 45% of married couples were related in Saudi Arabia.[1] While consanguinity is not unique to the Arab or Islamic world, Arab countries have had "some of the highest rates of consanguineous marriages in the world".[2] The bint 'amm marriage, or marriage with one's father's brother's daughter (bint al-'amm) is especially common"