Firstly I bet the majority of people in this thread who say "it's okay because you're not related" have never had a step sister, from my experience a step sister for intents and purpose is the same as a biological sister, you don't see them as being sexually attractive. That leads me to the next point, the Westermark effect, people who grow up with someone regardless of whether they're related do not find them sexually attractive, any failure of this normal developement indicates something is rotten in Denmark.