What the flying fuck.
Actually, though, there's a good reason why this shit happens.
It's said in literature that the 'ugh' reaction from incest is caused by spending a lot of time around someone growing up. So close friends and their siblings can give similar reactions (true for me), and family members you never see can be attracted to you or you to them (evidently also true).
It's incredibly messed up and primitive how our minds work, biology always assuming the simple cases for everything and not foreseeing changes in plans.