As for why MC was so trusting of his sunbae--he may have been 20 before transmigrating, but that's still quite young, and he was kind of traumatized by finding himself in a new body with no clue where or who he was. So if a manipulative psycho who is actively attempting to charm him is the first person who he establishes a relationship with, it's not implausible that he'd develop an unhealthy attachment. Considering that, I think the way MC handled the reveal of sunbae's deceit was pretty mature.

That being said... the weakest point of this is that it's transmigration. This is one of those cases where it's so unnecessary, and his character could've just as easily had the same experience going to live in dorms for the first time after how sheltered his family kept him (and being afraid people would avoid him after finding out about his family.) I'm so sick of reading transmigration where every 20 chapters I go, "oh, right, he's supposed to have transmigrated." Like he had minimal connection to the main characters, so the knowledge he did have had zero bearing on the actions he took--what was the point!?!