Muslms marry their first cousins. Imagine you and your spouse having the same two grandparents. Now imagine your grandparents having the same two grandparents, back seventy generations.
Read the details, just google "Islam first cousin marriage" or look it up on wiki.
Fully one half of all marriages in Pakistan are registered to couples who are first cousins.
Islam may make people insane, but when your family tree is a telephone pole, there might be more to it.
That’s very interesting, didn’t know that.
There's two main reasons for it:
(1) mahr(bride price): an amount that the husband is supposed to pay the wife (in reality, the wife's family) upon marriage. If the wife is a first cousin on the father's side, grandpa takes it out of one pocket and sticks it in the other.
(2) Clan loyalty: loyalty to the father's clan is very important. If the wife came from outside the clan, then there would be divided loyalty.