Fortunately, I do not have a sister, but I constantly have fantasies and I keep thinking how it must be to have sex with your sister. I have imagine that I'm doing my imaginary sister when I jack off. It's just so kinky to be doing it with your sister, cause you live with her.
I don't know why it's wrong, sure you are blood-related but dammit, if you saw your sister naked a few times, how would you feel? What if your sister throws the first move? What if she flirts with you? I guess I'll never have that experience.