I'm sorry...but did YOU actually read this chapter? Fumi was abandoned in a letter box and just because she's got it a little rough and is a bit more privileged she's got the right to spout whatever the hell she wants to? Is he supposed to just take it when he's like what 12?13? Maybe you don't know this but kids are more compulsive by nature and things like "it's never ok to hit someone" aren't always enough. The one being sexist here is you.
I have to agree with the anon on this one, whether that is how reality is in society in japan is irrelevant because just because a lot of people think one way doesn't make it right. You can't say "society in that area thinks that way so of course she reacts that way" as an excuse to not criticize the little girl because it is still a very old and upsetting way of thinking. It just means that those areas of society need to develop and gain some perspective. I don't personally think it was bad of him to hit her, i would never hit a girl and would have rather he DIDN'T hit her, but i don't blame him for doing it... She wasn't just making fun of him for being an orphan, she was also saying all his relationships are fake and making fun of his gay brother calling everything weird. Yes, Fumi definitely needed to apologize, but i can't be mad at him for what he did, just the act itself. I'm more irritated with the act of hitting a girl than with this specific scenario because she was more than asking for it...
This is a beautiful story and, you're right: to focus on what characters "should" and "shouldn't" do is a disservice to the story's authenticity and realism, and the masterful storytelling and art that Kii Kana has made for us. Regardless of how we might wish a character to act, the character is the person he or she is: in this case, an immature and insecure little girl acting out exactly according to her feelings. Little kids are egocentric, so they don't behave like saints. Adults try to muddle through to the best of what their upbringing and culture have shaped from them, and they don't always respond brilliantly and with omniscience either ;-). To get caught up in how these characters don't conform to our ideal expectations is to lose the chance of reading this amazing manga.
No matter what a person do to you, you don't have the right to insult them or hit them. Tha'ts why Fumi needed to apologize, it's not okay to hit someone even if that someone is really mean and despicable. And you can't be expecting that in a sexist country like Japan, guys don't say things like "you must never hit girls". I mean, people say that in "modern" countries too, so why are you surprised? IS NOT A GOOD THING, I know, but it upset me that you focus on the less important thing on the story, in the less important thing of this chapter. Seriously?
You can't forget that a comic/manga reveals the cultural aspects from where it comes, so the way the girl acts, even his stepfather, are "normal" in their society. Of course, not aceptable if you think of it, but this is exactly the point that the author is trying to convey. What is the real meaning of a "family"? If you teach to your son that it depends on the blood, we going to raise children like that girl. So probably she doesn't has a person to explain things to her, and she had hear rumors about the family of Fumi and can't understand it. Besides, her parents are divorced, she could be feeling the things from a perspective from where she lost all. Her father, her mother that never goes even if she promised, and now a stranger in his life. She hates the stranger, how can she undertands that Fumi is a strange that is loved?
Theres so much in this manga, please don't simplify it just like that. Sometimes I wonder if people really read, or just look at the pictures -_-
P.D: Sorry for my english~