I think I like my best guy friend?

I've known him since sixth grade. Him and I have been through a lot together. We got so close it got to the point where it was sisterly and brotherly love. We're about to be seniors and I've been noticing something different about his behavior. He's become more affectionate to me. He keeps giving me hugs, wraps his arms around me and he rests his head on my should or lap. And I have grown jealous, very jealous, when he speaks to other girls or when he does. I don't know what this is. But I do know that I can't like him or love him. He's my brother. My friend has been noticing this too and she asked me if I liked him. I told her no, but she gave me a look as if she knew I wasn't sure. I need help...