Two consenting adults, including relatives?

[Esquilax]

Well that goes back to the westermark effect. We're wired to be less attracted to people we grew up with. Some long lost relative might turn us on, but that person that lives across the street that's not a relative is overlooked for being too familiar.

I think I read somewhere about a royal lineage that almost died out completely because they kept raising their arranged to be married future regents in close proximity from childhood, so when it came time to further the family line, many couldn't bring themselves to do it, due to just that effect.