Good point.
Unfortunately, the “marry in” concept caught on throughout the Middle East and North Africa, regardless of religion. I recently read a book, “Jerusalem in the Time of Jesus,” by Joachim Jeremias, a German Biblical scholar. He mentioned that it was desirable for a priest to marry his niece, his brother’s daughter, because that way he could be sure of her pure bloodline.
My 5th great uncle was a Dutch Reform theologian who wrote a dissertation on why someone couldn’t marry their deceased spouse’s sibling.
http://www.henrylivingston.com/bios/jhl/marriage-jhl-p02-03.htm