Is it normal that i have crush on my step sister?

Hello. Well, it will probbably sound strange but I have crush on my stepsister. And I am a bi-straight girl. It's been almost two years since I realized what I am. My mother, some relatives, very few friends, my step father and my step mother's daughter know it.
I met my father's girlfriend last Spring and she has a daughter who's one year older. She was bi-curious but she is straight. We are very close, we talk about everything etc etc... Last summer, we spent two weeks in the same room, I've seen her naked, she has seen me naked, we've slept on the same bed.
When I returned to my mom, I couldn't stop thinking about her, her body, lying next to her... I thought I had got over it but I realized that I still like her. She doesn't have any siblings and for her I am a sister. I am confused...