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Well, it really depends.
When two individuals are equally messed up.
Let's keep in mind the Nii-san was the first to fall in love, the other followed after without his brother influence, but both are messed up, as long as they aren't toxic to each other then those stories with extremely messed up characters can only end with them together really.
If it was toxic then it's a different thing, I wouldn't want a happy ending (there was this webtoon I read that disgusted me because of the doctor literally kidnapping his patient who he "loved" and despite he told him to let him go meet who he really loved when he was about to die the doctor refused.
The other guy was just as toxic and I'm glad that they didn't met, but the doctor was equally disgusting, when you put kidnapping, medical malpractice etc. compared to cheating then the cheater in this case is the actual better person in THAT story and he deserved to meet up his dying husband.
Even because he was genuinely sorry, the MC lived in despair with the hope of seeing his husband.
Not to mention that he wasn't even able to see his family because of that crazy doctor.
Cheating is horrible, but the MC could have lived if he didn't kidnap him because poor baby couldn't handle rejection and decided to literally ruin the life of the person he "loved" when he was about to die)