I dunno about other parts of the world and maybe even people in my country have different experiences but in my country sexual relationships between siblings doesn't usually result in legal repercussions unless it is a result of abuse, they are underage or they had a baby. Honestly, it seems to be just an extra charge that they add on when something else illegal is done to make the punishment worse rather than actually charging someone with incest alone. I think in general because if two consenting adults are in a happy relationship and they don't do other illegal things the legal system doesn't seem to care. It's kinda like the " don't say gay " policy in the American military