It does seem to be traditional there from what I can tell and it's not like anyone gets the opportunity to mix with the opposite sex outside of family. But double cousins? That would be a little concerning for me. I hope they consulted a good geneticist at least.
I imagine that there are a lot of other cousin marriages involved in their family tree so they might end up sharing more than the usual max DNA for double cousins as well.
I understand it's cultural as well as practical in keeping all the power and wealth to as tight a circle as possible but it just doesn't seem wise.
if i am not mistaken these kind of medical tests are required in some Gulf states like UEA ,but not sure of Oman
Do you know, if they still practice the thing of the bride a) having to prove she's still a virgin and b) having to prove that she can have children? (not specifically in Oman, but generally in the Arabic royal world. I would imagine there're differences in the various Arabic vountries)
I know this was common in European countries as well.