I am in love with my 16 year old step-sis, but I don't know how to break it to her. What should I say to her?

Look at me…I’m so desperate that I’m on an internet Q&A site…

I’m 14 years old and I am deeply in love with my step sis. She doesn’t know, but for the past 3 years I have been infatuated by her beauty. I want to tell her how I feel…and I hope to get a good reaction. My mum met her dad 4 years ago. They are not yet married so technically she isn’t my sister at all yet, right? I have grown so close to her, I may as well be a fridge magnet…(her being the fridge =_=) For the past year I have worked up the courage to tell her how I feel and ask her if she would like to move our relationship up a bit…but I am afraid that she might feel disgusted in some way and not talk to me at all (or not for a long time). How can I tell her without her feeling as grossed out or sickened as she might be?