[MarMema]

Pretty much but not entirely.
The largest issue are the recessive diseases that emerge. Too bad the Muslim world has not seen more of this.

[Vigilanteman]

Yes, it is true that there are greater chances of recessive genes emerging in a first cousin marriage than otherwise. But the chances still aren't that great, sort of like the chances of matching 5 numbers compared to matching 5 numbers AND hitting the Powerball jackpot.

Keep this going generation after generation, however, and the odds multiply exponentially. i.e, if you lived 62,000 years and bought a Powerball lottery ticket twice a week, your chances of hitting the jackpot are pretty good.