@CodeGears The romance is between him and Hibino. His relationship with his sister is another thing entirely.
Rant mode on:
People treat this series far too harshly because it came when there was an outrage at other series presenting abusive female characters.
Every single reaction a character in this series has is a perfectly explainable reaction, and not a single character does anything out of spite or any other bad reasons.
Hikari treats Kenji harshly becaus she saw him doing something that can easily, easily be mispercieved as sexual assault towards his own sister. So her aggression is totally justifiable, because only we readers know that it was an accident. But, she's getting used to him and being much tamer as the chapter goes. His sister is also pretty tame, and she acts within their confort zone, and we can clearly see that this is how they act towards each other. But people still hate her, because they already came to hate characters that show similar trait, they project those characters onto her and hate her. Their cousin is a tomboy, and I understand why people dislike how she treats Kenji, she's the character that is closest to being an actual bitch due to how she always treats Kenji harshly with no reason, but seriously she's acting like a normal cousin, she may go a bit too extreme in her joking around, but she's not doing this to bully Kenji. She'd probably stop with some proper conversation, as hers are just jokes that go too far because she doesn't think things through.
Rant mode off...