Sadly not all societies/cultures have a taboo against incest:
"Couples who are getting married should be forced to have a DNA test first to ensure they are not cousins amid growing concern about incest within Pakistani communities, Britain's first Asian peer has claimed."
>Third-degree relatives (such as half-aunt, half-nephew, first cousin) on average share 12.5% genes, and sexual relations between them are viewed differently in various cultures, from being discouraged to being socially acceptable.
Relationships between cousins isn't always considered to be incest. It's more recent in the last 50-60 years, especially in USA, that cousin marriage is frowned upon based on misunderstanding the risk.
The other issue there is people who are related by multiple lineages. I know a family where the brothers married 2 sisters. Technically, their kids are cousins, but genetically they are siblings. Do enough of that and your genetic diversity of a community goes down to the point recessive diseases start showing up.
Relationships between cousins isn't always considered to be incest. It's more recent in the last 50-60 years, especially in USA, that cousin marriage is frowned upon based on misunderstanding the risk.