... is when a guy and a girl are very good friends, and extremely close, spending most of their time together. Eventually, the girl says "I think I love you", or in some way tries to make the relationship a physical one, and the guy simply doesn't feel about her that way, because he "Thinks of her as a sister."
Hence: The Sister Syndrome
Girl: Listen, Guy, I think I want to make our relationship between us a romantic one, I want to be more than just friends.
Guy: Ew! Sorry hun, I think we've got a case of Sister Syndrome!