Cause kids can be brats and its easy for them to act like this when they have super loving and supportive guardians. Fumi doesn't mean it and part of why he can even say it is because theres already so much love established between them. I agree he shouldn't say it and its too far in my book, but I think its meant more like I hate you screamed at a parent where its obvious its not how the kid feels
Now let put yourself just say in Fumi shoes. A elementary school kid on a conservative asia country. Before this event, he already feel unconformtable when somebody bring up that he is unrelated to Hasimoto family by blood as he afraid being ridicule by other kid for it. Now imagine, your brother being expose by media every day, by tabloid and tv for being gay. Being the talk of small town for living in his parent house with his boyfriend. He WILL be ridicule by his peer in no time. He will be perfect target for bullying as children is still cant learn to accept other people that difference from them and garpst concept of tolerance that good esoecially if they are not tell to by adult. I can see Fumi being called as okama brother (Okama is dergatory insuly for gay in Japanese just like Faggot in english) in his class or playground. Adult can probably handle this with cold head and understand that its not Mio fault. But 8 year old kid? I can see why he blame Mio for coming with his brother as source of all his problem being ridicule by his peer friend. And not to mention he is on his rebelious stage of life. I believe next chapter is gonna focus on that. I just want to say to not rationale non adult character action with adult character way of thinking.
I never like Fumi since the very beginning.
He rude to Shun, and how dare he treat Mio like that while Mio always nice to him.