In Star Wars: A New Hope, I noticed Han calling Leia "sister" twice. Also, he made it clear that he was in for rescuing Leia because she was rich who could well paid him. When did he romantically start to fall for Leia?
P.S. This question shouldn't be as controversial as this one: Did Leia know Luke was her brother when she kissed him?
because Leia wasn't a biological sister of Han as we all know.