So, in and of itself, two siblings sleeping with each other is not immoral if no one gets hurt (themselves or other people).
That’s the whole point of Haidts argument. Something is not inherently immoral “Just because it’s gross.” The reasons have to be distilled why it’s immoral, which may be because it hurts other people. A potential valid reason, which is based on logic, not “ewwww it’s gross.”
Haidt is simply trying to get people to logic their way through moral decisions, and not simply rely on gut reactions.
To further expand on “not telling a soul” so other people are not hurt, well, why do my family and friends get to determine if my actions are moral because their feelings are hurt? What if only 1 family member is hurt out of 15? Is it still immoral? Why does their inability to regulate their own feelings about X behavior make it immoral?
Yeah, it was an interesting thought experiment, and I suppose one could go quite deep with it and appeal to Kantian principles and so on, but as an exercise in getting people to examine their own intuitions and question if disgust alone can be a reliable barometer of morality, it does that very well.
Personally, I need to see a video of these two filthy, depraved sisters going at it hammer and tongs to really make my mind up.
