I get the feeling that her sister's another source of Hanabi's insecurities.
It's a common trait in abusers. Hanabi is insecure and has low self-esteem. She puts on a facade to act bigger than she is, but is terrified of others seeing her real self. I have no doubt she was enamored with Inchinose, but with her neuroses, she was afraid he'd realize that he was "too good for her". So she needed him to be beneath her and think that she was the only option he had. It went on for so long that her emotions became confused, and eventually her abuse became synonymous with her love.
I've experienced this kind of abuse first hand. I've been a victim, and I've witness the abuse of friends and family at the hands of their partners. We don't have to forgive the abuser, but we can still hope that they can move on and get the help they desperately need.