first cousins incest is perfectly legal so that's not a problem here.
A lot of people, e.g. many modern Americans, are squicked out by first cousin relationships even though it's legal.
OTOH for many societies, including 1800s England, it wasn't considered incest at all, just normal or even desirable. It's not like sibling or parent-child incest which are almost universal taboos, especially outside royal families with pretensions of divinity (and maybe a habit of not raising siblings in the same household.)
Arguably the tag should be used for relationships the characters consider incestuous and taboo. Or we should have multiple incest tags, one for the story perspective and one for viewers who want to avoid some definition or another of incest.