So does fucking your sister, if family and friends find out about it. That's why in Haidt's example he specifies that they agree to never tell another soul, and that's where I'd begin trying to build an argument as to why it's immoral. Is cheating on your wife moral if you never get caught? (Answer: of course 
).
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?
