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Deonepiece: Actually the part you mentioned happens later, and Falma's brother himself doesn't even know about it
yet at this point of the story.
The reason the brother is hard on Falma and his younger sister, is because he is like that (it has nothing to do with what you mentioned).
Remember, Falma's sister was already afraid of the older brother, because he trained and treated them pretty hard since childhood (its already been mentioned before).