Okay I've already read the chapter where it tells about yoen's brother. Why do you call him stupid? Where does it mentions that he was a bad brother to yoen? Reading those chapters again your theory makes sense, because it subtly highlights Yoen's brother, but I don't think he was a vad person, correct me if I'm wrong but didn't they explain that if the person who dies was a bad person he couldn't become a dream watcher? If he was bad he would become one of those like Aryka's red head friend, and also keep his memory, right?
During ch 26 - yoen is saying how his adoptive parents didn’t think their son was “academically bright” (which is why they got yoen so they could use him as an accessory to brag about being smart and perfect).
Even if he told the parents, that doesn’t necessarily make him an evil person, so he could still be good enough to not be a soul reaper, like arykas red head friend

Omg my theory based on the last chapter:
Since Aryka is the person that caught the 2 of them kissing, what if he’s the stupid, unhealthy brother of Yeon? So then, he told their parents about Yeon and Vince being together and got Yeon in trouble with them for not being perfect golden boy. Therefore, that ripped Yeom and Vince apart (the flashback seen with the parents and yeon yelling at Vince), leading to Yeon unaliving himself aka the reason for Vince’s guilt. And then since brother was unhealthy, something happened to him later on and then we was reborn as Aryka?
Ahhhhh this story line is so good I’m waiting for more chapters