Excellent point!
I think the difference is one breeds within the community, the other breeds within the family.
See also post #21, in that society if you marry your daughter off to a non-relative you have no control what-so-ever over how badly he mistreats her, you have some slight control (via familial pressure) over a brother.
Muslim rules of propriety also push a woman to marry a cousin.
She has to veil in front of her father in law and brothers in law, unless they are uncles and cousins.
If she lives with a patrilineal cousin, she’ll be able to visit her own parents without her husband’s permission if they all live in the same compound.
If she marries a cousin who then dies, she isn’t sent far away to live with her family while his family keeps the kids.
In Islam, you can have a “hanging wife”, whose husband doesn’t support her but won’t divorce her. Iran actually has charities for them. If she marries a cousin, she can appeal to her father in law / uncle and other relatives to help her even if her husband won’t.