The other girl in the flashback, seems like a bitch. Pretended to have this sisterhood bond with Hinako, but it was all a convenient lie for her. I'm not sure what "sisters" actually entails. It could be exactly as it's described in the story. But I could also see it being a euphemism for lesbian sex. Either way, a super shitty thing to do.
However, what Hinako did is far worse. She committed murder and suicide. Unless I'm missing something here, Hinako just pushed the other girl in front of an oncoming train. What the other girl did was cruel and manipulative, but in no way deserving of death. Unless we later find out that someone else pushed Hinako and the other girl, it makes Hinako a really selfish and evil main character. Having an evil main character can be entertaining; but it creates extra challenges for the author. One of which is having the right tone. And in this very first chapter, the tone is wrong.
Hinako and her giant friend, are presented as victims fighting against evil. While that is true in the case of this 2 dimensionally evil holy knight that tried to sell her as a slave, and kill the giant girl. It's not true in the case of the girl in the flashback; who we see at the end of the chapter. My problem here, is the self righteous tone of Hinako herself. That tone would be justified, if we hadn't seen the flashback of her murdering her former sister. But since we know Hinako is a murderer, it makes her self righteous tone, hypocritical.
Based solely off of the many other manga I've read, I'm going to guess that Hinako and her giant friend will be anti-heroes. And the other girl (former sister) that also came to this world, will be heroes, that are secretly evil. It's just really predictable. What I would prefer, is to see some remorse or guilt from Hinako, for the murder / suicide she committed. Hopefully I'm wrong. Maybe she turned down getting a cheat ability from the Goddess, out of guilt. As things stand right now, I'm not liking Hinako.