While there’s always more to say on the subject, you might be interested in these previous answers on the subject:
In popular culture and meme posts there's a stereotype correlating living in Southern states and incest. Is there any historical reason why this stereotype is a thing? Why aren't Northern American states associated with this stereotype instead? by /u/sunagainstgold
When and why did first cousin marriage become a taboo in many parts of the US, when it is not in most of the world? with u/yodatsracist .