Not allowing to marry outside tribes is totally regressive local cultural pracitices and have nothing to do with islam. I know ppl from arain clan do this arainon say bahir shadi nahi karni , awans have this thing too , some of them dont follow that but soem do.Its not sick and disgusting but their is definite co-relation between increased probability of defective child birth and continued inter-cousin marriages.
Yes, it is allowed but the concept of not allowing children marry outside is highly tribal (at least in our culture), which definitely is against the universal outlook of Islam.
The two most practising societies of this concept are Pakistanis and Afghans, two highly tribal and not necessarily model societies when it comes to practising Islam.
Mostly in pushtoon tribes from fata and some from south punjab have these practices where even men are not allowed to marry outside tribes where one cud get murdered for marrying outside the clan. ab yeh tau local ghalat practices hain , iski wajah say insan cousin marriages ko tau over all galiyan nahi day sakta. Now countrys law is weak and pathetic that it fails to provide security to those people who face threats from their fsmilies for having married outside tribes