My step sister snuck in my room to kiss me.

So, I usually stay with my grandparents so I'm not really around my step sister that often and I wouldn't say we have the closest relationship in the world because we usually go our own way. Recently when I went spend the night at my mom's I went to sleep rather early but was suddenly awakened to my step sister in the process of kissing me! I don't share an attraction to her even if we aren't blood related so I pulled away really fast and we were both just speechless. We haven't so much as looked at each other since because she's been avoiding everyone. Our parents our worried but I don't think I should tell them the truth because I think I should be the one to ask her. Our relationship again was never extremely close because we were hardly around each other so I'm not sure if she's really developed some infatuation with me or it's something else. I don't really know how to go about it. I'm probably just as confused as she is.