Wat if I like my step sister Wat would I do?

Have a talk with her in private sometime and tell her how you feel. You don't know if she feels the same (odds are she doesn't but who knows?), but nonetheless, this will evolve into a heavy burden if kept a secret and she'll start noticing you behaving strangely, so you should come clean with it as soon as possible. Now, if she'd actually start resenting you for how you feel (especially if you don't attempt anything beyond being honest) then she's narrow-minded and needs to be left alone for a while in order you process the fewfound fact. Hopefully you can still maintain a good relationship, or else she's no one to waste energy on.

ummmm.... it isn't like she's related to you....

go munch that box!