Actually I think there is a psychological blockade to prevent incest but only if you grow up with someone. I have two half sisters. One shares the same mother the other shares the same father.
I grew up with my mother and with one sister. She is very good looking. But I grew up with her and even though I know rationaly that she is pretty and we share just half the genes I feel absolutely not attrackted to her.
I couple of years ago I had first contact to my real father. So I visited him, I arrived at the airport and saw an extremly hot girl, after a couple of minutes I knew it was my other half sister.
I still have problems to see her as my sister. Its mainly due to the fact that I met her first time with 21 and see her once a year.
After having that experience I know that atleast if you share just one parent and dont know its your sister you could do her. Before that I thought there would be some kind of natural blockade like you wouldnt find her attractive.
