Here are the spoilers, I got lazy so I'll just put it here . I'll breakdown the spoilers based on a linear timeline of the events and separate it into parts. There are some events that I forgot but I'll list down what I can remember.
Yujin & Winston has good relationship when they were young, tbh they are one of the couples in Zig's universe that has a good relationship from the start of their relationship, if it wasn't for the events that happened.
Childhood and First Meeting
Harold (Winston's Dad) took Yujin from Korea after his parents died. Yujin's omega parent was Harold's First Love/Ex, that's mainly the reason why he took YuJin in but he was not officially adopted, he was more of a ward. Growing up, Yu jin was taken care of by Servants & Harold, he was given food, shelter, books & entertainment but no formal education. He live alone on the guesthouse, where his only visitor is Harold and the servants, so he kinda crave Harold's attention. YuJin see him as a father figure, though he was afraid to disappoint him so he studied very hard. The Campbells (Mrs Campbell and the siblings) see him as a pest but they never really go out of their way to torment him, they just ignore him and constantly talk shit about him when he came out in a conversation. Winston, as the youngest, never really knows what was going on, he just know that they have a guest in the guesthouse and was really curious about him.
Winston met YuJin when he snuck out, he was Yujin's first friend. Winston would often snuck out to accompany YuJin and tutor him, using horseback riding as an excuse when asked. These meetings was not known by anyone in the estate, it was a kept secret by Winston and YuJin. Later, YunJin manifested as an omega, at the same time Winston manifested as an eztreme Alpha. While both of them was unconcious, Winston was shipped off to England to study, so they never really get to say goodbye. Since, no one knows about their relationship either, Yujin was not informed and often wonders why Winston never visited him anymore.
Around this time, Harold's treatment of Yujin shifted, he started sexually harrassing him. He started smelling his pheromone, making him remove clothes and started touching him. Yujin resisted at the start but Harold slapped him so he grew afraid of Harold and he was afraid of his visit. He tried to ask Mrs Campbell for help but he was slapped and turned away. At this time, YuJin is still a minor, probably around 13 to 16 and hasn't really fully manifested (hasn't entered his heat yet). The Campbells actually think that once Yujin turns 18 their father would take him as a pheromone partner, since Mrs Campbell is a Beta and can't help when Harold entered rut.
Early Relationship (timeline probably around 16 years old to 18 years old)
When Winston finally got home, after years of being separated, they finally meet again. They both confessed to each other and is finally in a relationship. Yujin also managed to tell Winston what happened and Winston was furious. Winston assures Yunjin that he will do something about it, unfortunately, since he is still young, he can't do much (they are probably around 16 atp). Using the excuse that he wanted to learn the ropes in their business, Winston manages to convince Mrs Campbell to let him stay.
Winston leaked a money laundering scandal to the press relating their company to distract Harold so he can buy YuJin time. Due to this, no one was paying attention to YuJin and since Winston was expected to be the next head, it falls on him to keep an eye on Yujin while Harold was busy with the scandal. Winston would often snuck YuJin out of the house, he wanted to show him the world, and discover new things, find something that interest him, since Yujin didn't really go out of the house. One time, Winston manages to take a vacation with Yunjin outside, they use Yujin is sick and contagious as an excuse, and since Winston is an extreme alpha, he can drop him off at the hospital on the way to the villa, without getting infected.
While staying at the campbell villa, Yujin met some of Winston's "friends" and Evelyn for the first time, the "friends" crashed into the villa and threw a party. Winston intoduced Yujin as his boyfriend and everyone was very friendly. While Winston was getting some drinks, the "friends" told Yujin that Evelyn and Winston are ment to be "married" one day and that he might be just temporary (apostrophe cuz it was official but was only known within the circle cuz of their family social status and was expected), hearing this Yujin got upset. Evelyn told Winston that Yujin was upset about something and told him to go talk to him, Winston found out what happened and kicked everone out. Yujin and Winston talked and he confessed that he was serious about him and he just viewed Evelyn as a younger sister and nothing more (btw Evelyn was not part of the group that told Yujin that he was temporary, just wanted to clear that this girl is innocent and she just gets dragged around into shits cuz of what is expected of her due to her family and the campbell's relationship)
Living Together (around 18 years old to early 20s)
After a year or so of living peacefully without Harold, Winston & Yunjin got caught, I don't remember how, but when Harold found out he was furious that the person he was "protecting" got stolen by his son. Anyways, Winston confessed that he was serious about YuJin and He and YuJin left the Estate. They started living together in a one bedroom apartment, while living on Winston's paycheck. Atp, Winston was no longer a part of the Campbell group and is working to keep themselves afloat, since Yujin doesn't have a formal education he can't get a job on his own so he mostly stayed in their apartment.
Btw, Winston wants Yujin to discover himself atp, and find an area of interest so he can study or proceed with his education, he doesn't confine him at home and he wanted to Yujin to make friends so he wouldn't be alone. Unfortunately though, since Winston was often busy at work and often comes home late he never gets to spend time with Yujin, Yujin became lonely and resorted to drinking to ease the loneliness. Their relationship is still pretty good but Winston is getting worried about Yujin mental/emotional state that's why he encourages him to go out and make friends.
After years of living together, Harold reached out to Winston and invites him back to the company and the estate, Winston agreed to go back into the company to give Yujin a better life but refused to go back into the estate and instead moved to a different apartment, one that is owned by the campbells. Unfortunately, since Winston is still busy so Yujin is still lonely. Atp, Winston's brother (the druggie) started visiting Yujin, they have an amicable relationship where they would talk together and Yujin actually stared to like him since he is the only one in Campbells (besides Winston) that treats him well, however the druggie did use Yujin's apartment as a "hideout" where he would do drug and invites friends. Winston doesn't know about this visit and Yujin didn't tell him, he just know that Yujin often have friends over.
Separation (around 18 years old to early 20s)
The druggie son's visits was actually used by Mrs Campbell as a way to stage an accident for Yujin. Mrs Campbell use the druggie son's connection to hire some people to break into Yujin's apartment and record him doing things. Winston, fortunately, stopped everything before it could happen but he got shot and was put into a coma trying to protect Yujin. Although Winston was out of danger he still has regained conciousness (from what I can remember he was in coma for 3 months or more). Yujin tried to visit him in the hospital but was always barred from meeting him. He managed to sneak inside for a bit, during this time, Winston regained conciousness, though he was at a dazed and in rut state. Yujin helped him released his pheromone and Angie was created. Now, in this omegaverse, Alpha usually lost some of their memories while they are in rut add the anesthetic medicine, that's why Winston doesn't remember.
Anyway after this, Yujin only ever managed to see Winston again when he actually regained full conciousness and it was because Winston insisted on seeing him, this was also orchestrated by Mrs Campbell to make them look good in Winston's eyes, that they are taking care of Yujin etc. etc.
Yujin extremely worried about Winston, approched Harold to plead/made a deal with him to let him see Winston. Harold made arrangement to see him in a certain time and place so they can discuss on how they will proceed. At the same time, Winston is also worried about Yujin so he arranged to be released early from the hospital. He made his way into the Campbell estate (Delights) and saw Yujin under Harold. This is all orchestrated by Mrs Campbell and the family, they wanted to make Yujin appears to be having sex with Harold and fool Winston into thinking that Yujin is a loose person. Mrs Campbell didn't really care if Harold raped Yujin and stated that it's actually better if he did. Btw, Harold was in on the plan but it was only to apease Mrs Campbell, he didn't touched Yujin. Yujin was drunk on pheromones at that time due to a drug they gave him, he mistook Harold's pheromones for Winston's (I'm not sure how, probably due to genetics Harold and Winston has slightly the same scent).
Anyways, this whole staged event (Yujin doesn't remember the whole event, he just remember drinking tea, falling asleep and waking up in a different place afterward), Yujin was cast out of Delights, he was waiting for news but he only got wind of the news that Winston is getting engaged.
He went to the estate so he could talk to Winston. When he arrived everyone was in a festive mood. He requested to see Winston and was guided to a room. He was able see Winston (he still hasn't fully recovered and currently walking witha cane) and of course, Yujin was very worried. He asked him how he was, why he didn't contact him and if the news about the engagement is true. Winston told him that the party was actully due Winston's engagement with Evelyn. Yujin plead with Winston to let him stay by his side, he doesn't even care if he became a mistress atp. Winston was flabergasted and just pushed Yujin away (not literally). Yujin then confessed that he is currently pregnant with Winston's child. Now, in this omegaverse, Extreme Alpha has complete control of their reproduction, if they don't want to impregnate their partner then it would not happen, this usually only happen when the Alpha is in rut (I can't explain it in details but Keith/Dr Steward explained it in Kiss Me Liar), and since Winston doesn't remember the rut during him time at the hospital he didn't believe Yujin. Around this time, Winston was already "brainwashed" by Mrs Campbell that Yujin is a loose person and was only with him for his money since Yujin doesn't have any means to support himself, and would probabaly use the reason that he is pregnant to latch himself to Winston.
After this event, Yujin travelled away from delights using the little money he had that was given to him. Since he doesn't have any education he was not able to get a job. HAt one point after work, Yujin got into a car accident while inside his coworker's car due to this he went into labor early ande was homeless and wandered around, collecting scraps to get by. At one point, he was even arrested due to stealing a person's wallet (I'm not gonna get into more details cuz I always cry atp of Yujin life). A police, Sam, helped him out to get out of jail after finding out that he's pregnant and the reason why he stole the wallet. Sam helped him too his feet until Angie was 1 year old and then Yujin moved out of Sam's apartment. At one point during her pregnancy, Yujin got into a car accident while inside his coworker's car, due to this he went into labor early and was unable to get pregnant in the future. It is also due to this that he developed his fear of cars, cuz he almost lost Angie.
Winston, Yujin and Angie's Relationship etc. (3 years after the event)
Fast forward to the current events. Winston and Yujin's relationship are in shambles. Winston believes what he saw and YuJin denies what happened. He doesn't trust Yujin anymore and barely listen to him and often accuse him of being a sex crazed person. Yujin, although he longed for the past, he doesn't really expect anything anymore. He didn't deny the accusations that Angie is Harold's daughter just so Winston won't have any hold on Angie cuz he is afraid that Winston would take Angie away from him. Winston used the marriage to tie Yujin with him cuz he doesn't want Yujin to run away again.
Winston and Angie's relationship is non existant. He saw her as an "affair baby" and isn't fond of the idea that Yujin had a baby with someone else. He treats her well enough for her to live comfortably and treated well inside the estate. Angie doesn't like Winston cuz of his pheromone, probably cuz Winston is usually hostile towards Yujin. She also saw Winston as an enemy and applied "Daddy doesn't like him, so I don't like him too" . Later on in the story, Angie got lost in the estate grounds during a thunderstorm and was found by Winston, according to Angie, he stated liking Winston around this time but continued to pretend hate him cuz he is "Daddy's enemy".
At first, in order to gain Yujin's trust, Winston started getting into Angie's good side. He started buying her toys and princess dresses, playing tea party, throwing a Halloween Birthday Party, and even offering her stocks when Angie wouldn't budge. Winston also though Angie how to swim and even ask Keith for advise on how to gain Angie's trust. Overtime, Angie grew close to Winston calling him Uncle and sometimes Winnie (a nickname used by Yujin too when they were younger). Yunjin was conflicted with this, although he is glad that Winston treats Angie well, but since they are going to run away eventually he feels that it would be unfair to Angie.
After the runaway arc, Winston found out the truth thanks to them finally talking and thanks to Georgina (Winston's niece) providing proof that Yujin is innocent. Winston went on a revenge spree to his family including himself.
The End of the spoiler

Angie deserves the world istg if Winston does anything to try and hurt her