It’s all in the small things. The way he looked at them while they ate, how his wife feared him, how Demetrius was the only one not uncomfortable (at least seemingly), etc
You're not alone dw. I'm bad at reading rooms too
The father is very calculative. You don't need to be a mind-reader to know Damien wants his approval. Though he has the eldest son to mold, keeping a reasonable distance/closeness with his "spare" is handy for now. The mother is old enough to see who he is, even of she doesn't see everything he does, so she won't fall for it.
Why did I lowkey think because of how the dad just said the dinner was worthwhile bro would end up being a cinnamon roll later in this manga... Then I saw the comments and I think I misread the room