The bit you made above about how his environment with his sister and her friend sort of made who he is today is fine, but I think what I quoted above is moreso the sister "criticizing" the brother about him calling the new girl by the name "Seria" and him revealing the LINE sharing so nonchalantly and openly in front of Hinako. At least, that is how I saw the "He really doesn't learn..." line.His sister has absolutely no right to criticize him. He is the product of her own making.
Of course, how he grew up may have played a part as well.
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