Responses
Yes, the backstory from the game is, when she was young, there were some conflicts between Onigashima and another gang. To protect both her mother and her, their father sent them away to somewhere safe. But along the way, something happened to her mother and the father lost track of Hinako.
It just seemed so random in the beginning for him to tell her she is his sister. He just stumbled upon her in the street and used the resemblance. I strongly hope they are siblings and that way she has gained 2 brothers after growing up an orphan but it just seems unlikely and that makes me sad.