I think i should post my two cents. I definately agree with what most of you said here. Especially about the three of twins in Onegai Twins never having met before. Let say for example, you grew up living together with a person who you thought was your sister. Then many years later, one day you two find out that you guys aren't related at all. Would your brother and sister relationship dissippate? I highly doubt it. To me, Incest is more of a family oriented thing. If you grew up as brother and sisters, whether you are related or not, you act and play the role of siblings. So a sudden discovery that you two are related shouldn't change the role you two had for many years. In that situation, if they eventaully became lovers therefore, would be pretty.... abnormal.
But in another situation, say you never met someone before, and when you get to know him/her, and gradually, you fall in love with each other. Your love continue to grow until one day you discover that you two are actually full blooded siblings. Then, would you disregard your current feelings for each other? Perhaps, but it would be very difficult thing to do i'd assume, even with the ackwardness of the new relevation.
Of course, the worst case scenerio is in Angel Sanctuary. Where two person play the role of siblings AND they are full blooded sibilings. That's why it's hard for many people to accept that kind relationship, even me whom i consider pretty open minded.
