my parents are divorced. i'm nearly 16 (assuming you were going by legal ages, thats the 'recommended' age of consent in scotland.) if i found out i had a step-sister (and because my mother is now using internet dating its a possibility) and they were hot... i'd probably do that. (though my plan if my ma dates a guy with a hot daughter i'd bang her first.) hell, i even if she were my half sister. and if a dream is anything to go by, my cousin (in a threesome.)
its actually a phenomenon because of reverse sexual conditioning: you're around someone for the majority of your life when relationship boundaries are set up, so basically you see your brother/sister every day and see them as family-not-fucky. you dont see them at all, then you dont get the reverse attraction effect.
put it this way: once i had a fantasy (it was a dream and i was fairly young) that i married my best friends hot sister. this was also a situation where i created myself a sister who he ended up married to and i was even more attracted to his sister.
although its not a blood relative, its technically breaking a social taboo. implied full incest is hot to some people.