My wife and her sister look like twins, even though they are half siblings. My wife is blond/brown and sister is a brunette. That's the only difference.
But I never ever thought of her that way. It would be gross, I think of her like my sister, and it's incestuous. I don't mean gross in a moral way, just being a good boy, or whatever. It's just wrong. It would be like kissing my sister.