Well.....
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For the TLDR- Kwon eventually bonds with Sejin and Sejin gets all the horrid memories of their lives together in their first life. He ends up leaving Kwon. After some months apart and therapy, he starts seeing Kwon. They are bonded so breaking up isn't really an option.
A more detailed spoiler
Kwon eventually bites Sejin and bonds with him. Because bonded partners share emotions and memories, Sejin gets all off the memories to their first life together. He gets offered to Kwon and Kwon treats him horribly. He's dismissive... puts him in the smallest room and a tiny bathroom with just a standing shower. He's denied a tub to take a bath. He stays loyal to his family desperate for that affection & acceptance, but ends up the sacrifice getting sent to jail for Haeshin's downfall. Kwon eventually gets him out. He steals Kwon's work info giving it to his dad. Because of a genuine mistake/miscalculation on Kwon's side while he was out of town- telling his brother he has a gift at his home waiting him, Ijung goes to his house. He meant the evidence to put Ijung in jail for raping and abusing omegas. Ijung goes to Kwon's house and finds Sejin in heat. He thinks Sejin is the gift and brutally rapes & knots him. Because of Ijung's taunting during the assault, Sejin is convinced Kwon sent him on purpose to rape him and starts to break down. The house staff had been instructed not to monitor Sejin so they did not check on him during the assault. Kwon gets back home 3 days later, while in rut, and smells his brother's pheromones and sees the marks on Sejin. Sejin doesn't say he was raped. He's more like, yeah. It happened.. because he thought Ido sent Ijung to do it. Kwon, fueled by his rut, snaps thinking Sejin voluntarily slept with Ijung and brutally rapes Sejin as well. He also knots Sejin and forcibly bites & bonds with him. He gets all off Sejin's memories and emotions and realizes how much he misunderstood Sejin's upbringing/situation, how horribly he's treated him, and essentially sees how his brother raped him and how he himself raped him from Sejin's point of view. He feels horrid but it's too little too late. He does turn over the evidence he'd been collecting on his brother and he's put in prison. Sejin is completely mentally broken. He brings Sejin into his room, instructs the staff to feed him, gets him IVs if/when needed, and visits him at night but Sejin is a shell of a person. 3 months later, Sejin realizes he's pregnant. Unsure if the father is Ijung or Ido, he goes into Ido's study and gets the gun. Ido walks in just as Sejin puts the gun to his chest and pulls the trigger. Ido sees Sejin kill himself. It is too much for Ido. Ido eventually dies too but is reborn with his memories of his previous life (that's why he said he hasn't acknowledged Ijung as his brother since he was 5). Sejin is overloaded with all of this hitting him after bonding with Kwon Ido. He's upset Ido never said anything, let alone bonding with him knowing all that happened and that he would get these memories. Ido had been living in a way to atone to Sejin but never expected a real relationship because he felt he didn't deserve one. Sejin leaves Ido. He's pretty well off since he's running his own company that Ido gave him. Gets therapy. But because of the bond, he's always aware of how much Ido misses him and feels like shit for everything. Sejin starts to feel upset thinking Ido is getting over him because Ido seems to be feeling happier. It ends up being Ido started feeling happy because he knew he'd see Sejin in a meeting. Sejin does eventually start seeing Ido again. They were bonded so can't really break up. Oh.... as you know I'm the current time-line, Ijung is missing. Ido is responsible for that. He doesn't bother putting him in prison. He lets Ijung experience what his omegas victims experienced. It's not said how it happened- self or from Ido's people, but Ijung is eventually found dead.
I don't want the train wreck that's coming, but I want to get it over with