I get what you mean, I really, really do but I'm going to give them the benefit of the doubt that therapy still isn't that widely accepted or used in mainstream society globally...I'm going to go on a limb and say particularly in Japan when this manga was written.
Also Rui's been acting like this for a while... They probably just thought he was really concerned with acting 'proper'...they probably didn't connect this to his trauma and fear of abandonment.
And the pregnancy news
It doesn't matter how good your intentions are when the impact they have is harmful.
If you're going to take in a kid who's been through something like that, and you've done no fucking research, and just ignore everything, then you're absolutely responsible.
parents are responsible for their children and what happens to their children, and this is shitty parenting. even though they love him and they've never abused him, they aren't meeting his needs and they never have.
The most basic of internet searches would have told them better, but they apparently didn't do even that.
Honestly like the adopted parents suck. They saw him become this perfect robot, never took him to see a therapist, and then didn't do any kind of reassurance when telling him they were having a biological child.
like did they put any fucking effort into this besides just okay we'll take you home your our kid now? Did they remotely consider the psychological trauma and long-term impacts that would have?
Honestly this is shitty parenting.