It looks like it's more of a frog in a pot of water type of thing. Daichi knows something is wrong with Jun, he just doesn't know what.
And from how Jun interacts with people ( his gf, daichi's friends, daichi himself), you can see he is good at easing the atmosphere (I hope I am explaining this well). There is something manipulative about him (we the viewers can see) but Daichi notices too but he is naive and doesn't pay it much mind.
I think you explained it perfectly.
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Is Uke seriously considering his sister's bf? Like, no matter how fucked up someone is and tries to get me in their trap, I'd never go for a sibling's partner.