May I ask where you got this from?
The term "Arab" is very vague. It can be the some 300 million people split over 22 countries or it can be narrowed down to the less than 30 million people in the Gulf (Saudi Arabia, Bahrain, Qatar, UAE). However, there is some truth to what you said regarding marriage between cousins and it is not uncommon but it's not encouraged either. It is also worth noting that the staggering majority of cousin marriages in those societies are between second cousins. Also, in some tribes the heads of the tribes have to marry someone from the same tribe. It is common in tribal societies like those in the Gulf and Yemen but is rare in the Levant (Syria, Lebanon, Palestine and Jordan) and North Africa (countries such as Egypt, Morocco, Tunisia and Algeria).
It is also worth noting that is discouraged in Islam, the religion that dominates the area. The prophet of Islam encouraged his followers to marry from different tribes to forge ties between those families.