A thing that in some cultures is expected and revered, and here (United States) is vilified: sex with your cousin is seen in much of the United States to violate biological boundaries and spheres of closeness. To keep family, clan, tribe or lineage strong, cross-cousin marriage is/was a common pattern in many societies.

Take it from someone who studied anthropology, cousin marriage makes a family tree look like a tapestry. Margaret Trawick illustrates this quite literally in Notes on Love in Tamil Nadu, lying traditional weaving patterns next to a diagram of an idealized family structure (repeating, nearly self-contained).