This and the three posts above me are all messed up.
Throughout your whole life, you'd know that you effectively "banged your sister"
But I mean seriously. You're not related.
Imagine it like this. Say you're dating this girl, and no one really knows about it. Say you've been having love. And for this purpose say you're like 20 something so you don't really see your parents outside of holidays and occasions.
Now say they meet the mother/father of the girl you're dating, not knowing their kids are dating, and get married. You are now dating you "sister." I mean, why is it within literally one act of a legal contract between your parents does it suddenly make something that wasn't incest, incest.
Not to mention incest between cousins was super common a few hundred years ago, and while admittedly I think it's creepy, at one point in time it was accepted and practiced. So my opinion is, so long as their is no blood relation, I don't think it's incest.
Edit:
What if you are going out with a girl and you have a single mom and she has a single dad and they meet through your guys relationship and fall in love and get married and then your GF becomes your sister.
Well stuff, what he said.