Comment by Galactic Navy Officer on Another World Territory Reform ~Starting Public Works with Earth Magic~ - Chapter 24.1

@Galactic Navy Officer On a separate note, treating slaves as slaves is a learned behavior, like other forms of prejudice and bigotry. While kids will go "Ewww" at each other for the silliest things sometimes, it doesn't stick long-term unless it's reinforced. 

(This does not include when a child is showing aggressive or vicious behavior and the kids quite rightly want to avoid them.)

@Galactic Navy Officer Anxiety and loneliness. The thing about her promise to their mother about protecting him and failing to do so, together with the need for her to have the only real blood connection give her some peace of mind is a big problem for a girl her age. She spends most of her time studying all the time within a house that feels... unconfortable at times. I mean, Step mother is amazing but... it can't substitute the blood relation with her own brother after all. The fact that the mc is working a lot even though he is only 6 must be really alien to her. 

That and dad is probably too busy to give proper attention to her... she must have suffered a lot. Her behavior is quite well understandable.