It’s more of an issue with endodgeny than actual incest. It’s common in isolated areas areas. When there’s a small population with few new people moving in, eventually you get several family groups that are connected several ways. It still can result in undesirable traits getting passed down that otherwise wouldn’t be expressed. Think the blue people of Kentucky. There’s many communities that experience endodgeny, not just Appalachia. Some are isolated geographically, like PEI in Canada, or socially, like the Ashkenazi