This post actually seems somewhat provocative with your avatar and usertitle.Was just going to say that a step-sister is not a blood relative, so I wouldn't say there's anything totally wrong with hooking up with one if the feelings are real... and since she's not even your actual step-sister that's doubly true. It's unconventional and therefore a little more complicated than normal, and carries a stigma, so it should be avoided unless you're very sure moving forward. If you are sure how you feel about her and you feel about her strongly, then I would approach her in such a way as to not upset her since she might not be as into it as you. The way you talked to her about it so far seems like it went alright.
Aside from that I agree with JC:
Just be careful in your analysis of things, and since liking her wouldn't be wrong but is more sensitive than it otherwise would be, I would take care moving forward. But don't ignore your interest your feelings are strong or become strong. Just be as honest as you can with yourself about how you feel and try to critically analyze your reactions to her. This might be hard if you're inexperienced with girls and attraction, as JC cautioned.