The abuse of the Aono brothers is morbidly fascinating. All of it stems from their mother being a unfathomably immature person. She has high demands towards everyone around her, yet is incapable to provide anything herself.
Ryuhei receives affection not because him being a calm, attentive child makes him more easy to love. In fact he was molded to be alert to his mother's emotional distress. Reacting in worry, and taking care of her first got him praise and her affection. Only... the compliment of him being like his father was a double edged sword... as in most recounting about his father was about what he provided to his wife. Therein lay her adoration for him. (Which bordered on parental incest comparing the son to his father in both providing the same services to the mother's needs.) In that sense, her affection is conditional to the comfort and care someone brings to Mrs. Aono first.
Only problem is, her boys still were children. They need to be taken care of first as they're deeply vulnerable. Whereas I see Teppei's behavior as rather normal and typical behavior of a kid his age, expressing comfort physically, having strong reactions to distressing feelings like anger or fear, disliking to get scolded testing his limits with his parent, playing silly games, it is so telling to me that his mother deeply despises him for that behavior. Because Teppei's behavior is typical for his age, all that he does is what she does too. Only... it's a developmental stage for Teppei. For Mrs. Aono it's childish behavior which is not appropriate to a grown woman who has the responsibility of raising two kids. She puts her feelings first, is more scared of getting (eventually) reprimanded by her family in law for her messy life situation, finds fighting back at pushy clients to hard, is easily scared by even small challenges, is rather passive, rarely if never proactive. Even though she screams at Teppei for hating weaklings, she herself never was a strong person. Her dislike of Teppei's childlike behavior is projection of her own childishness. As well as her inability and unwillingness to overcome her severe limits. In so it's fascinating how she takes Teppei's behavior as indicator of a deeply malcious and mal-adjusted personality - of course without an ounce of self-reflection. And yet, despite being the despised child who follows his child-typical instincts more than Ryuhei, Teppei finds the freedom to actually retort at his mother what burdens both of them.
Just the juxtaposition of both boys who're more alert and perceptive than their mother against her mother who's a grown adult but absolutely not capable of adulting.
With a grown body and status as mother she also wields a scaring power over her boys. She's like a tyrannical child in how she exerts her power. She wants to receive comfort first, everyone should obey her when she demands it, and hates getting asked to make an effort first. Her strong emotional reactions to distress are absolutely disproportionate. An obvious sign is her physical abuse of Teppei anytime he plays around like the energetic kid he is, or tries to receive physical contact because she's still his mom. He becomes overwhelming for her? She needs to beat him back proportional to her flaring up anger. But with Ruyhei, she reacts more perfidious. Still childish but with him she exerts emotional control. Ruyhei asks her to be nicer to Teppei whom she depises? She really doesn't like Teppei but Ryuhei asked her to do something she doesn't want to. So she makes Ruyhei pay by playacting favourites.
In that psychological power she holds over him, Ryuhei caves into her demands in order to not loose her affection. In so he learns the he needs to be useful and dependent on. It's the only way he can receive affection. So he follows a script to receive love. So in the logic of a child only experiencing love as conditional, he follows his mother's logic who of the two boys "earn" love or are "rightfully" denied love based on what they provide for the parent.
