I need help figuring out my feelings for my best friend?

I'm not 100% sure on what I'm even asking at this point. My best friend is a girl, and I'm a guy. She is that most amazing person I've met in my entire life. She's funny, smart, nice, and absolutely gorgeous. But here's the thing. Half of me loves her like shes the sister that I always wanted. She loves me like her older brother, her parents love me like a son, and I'm essentially part of the family. I legitimately love her as much as I love my own flesh and blood. She is for all intents and purposes, my sister, and that's how I truly want to feel about her. On the other hand, the other half of me, is in love with her. HARD. Like, she's what I think about when I'm not doing something. I love her desperately, and would do anything for her. I feel that there's a fine line between loving someone like your own flesh and blood, and being in love with someone, and with her, I'm riding the Hell out of that line. I don't know what else to say. Sometimes when I look at her, the only thing I want is to be with her. Then most the time, she's like the sister I've never had. What is my problem? How do I get over those weird residual feelings for her that I guess never quite went away?