Is my boyfriend in love with my sister?

I have been dating my bf for almost two months. He is a very sweet and kind guy. I met him through a group of friends. Before we started dating, it was no secret that he really liked my little sister; its rumored that he even asked her out, but she was not interested in him. A few months go by and I am single. My bf and I become really good friends and that blossomed into a romantic relationship. But sometimes, I feel like he takes more of an interest in our conversations if its about my sister. Before we started dating, I asked him if he still had feelings for her and he said no, that ship has sailed and that he cares for me now. But he left his cell phone over here the other day and I went to look through the pix just to see what pix he has of me and he has none! but he has a pic of my sister. IDK how to feel about this. My sister says she is not interested in him at all and my bf says he isn't either but why would he have a pic of her and none of me? What do you think I should do?