If you really can't imagine that someone would be grossed out by the idea of screwing his sister, then I can direct you back to my other example of the pretty girl with an ugly set of values.
Unless it's simply that you won't accept any other answer than "yes you're right in fact it's just that I'm a dirty [whatever]-hater". In which case the whole discussion is pretty pointless, because there is no real interest in what people think, just a way to find pretext to put them in the bigotbox.
No, I'm not trying to put you in the bigot box.
I'm grappling with the distinction between "unconscious attraction" and the various conscious responses that overlay them. If the attraction is truly "unconscious" then I don't think it is subject to change. The visual cues are all still there. I went through a period after my first divorce where I really hated my ex-wife. I mean see-red-seriously-consider-murder-on-the-spot level hatred, which she did everything she could think of to exacerbate. But it was annoying that any time she shut up for a minute my testicles would say "she's really hot, you should calm down and try to get some of that." The
attraction never changed.
I'm thinking that the responses you describe come from having internalized taboos. That may have been just as unconsciously, and I am not trying to fault you for it. I'm just thinking that it really is distinct from attraction or lack thereof.
So, question for speculation:
You meet the girl. She is "attractive," meaning that she activates whatever set of visual cues 'do it' for you. At some point you realize she is the long lost product of a tryst your parent had, so, your sister. We know your response there, but I think that
she still meets your particular set of visual cues. So to me what we have is a case of attraction that you are determined, consciously or unconsciously, not to act upon rather than a lack of attraction.
Now, a new revelation. Long lost sister, as it turns out, is an aspiring actress and when you met her she was made up and costumed as a stand in...a position she was chosen for because of her remarkable semblance to Hottie Actress X, who she introduces you to. Do the visual cues of Hottie Actress X attract you, or have your tastes been altered by the experience?