All these ppl whining about them becoming step-brothers. Maybe you should’ve read the sy...

They don't get that even real people in the same shoes as these two would NEVER think of each other as siblings. They're WAY TOO OLD for that, and they've already had sex. An adult who's already working and supporting themselves is completely different from a minor still living with their parents, or even from a college student still needing their parents to pay their college fees and support them financially. Kids living with their parents and college students financially dependent on their parents are stuck accepting their parent's choice whenever one of them decides to remarry, so they usually have no choice but to get to know the new "step dad/mom and their kids" and try to get along with them as much as possible. But when you're already a working adult paying their own taxes, you think of both you and your parents as separate people who love each other but still want to carry their own separate lives. So, if your widowed or divorced parent suddenly comes to you and says, I'm marrying X, and X has a son/daughter, you don't instantly think of X as family. To you, they're just the son/daughter of your parent's new lover, and that's it. Chances are, you're not even going to live with your new step parent and sibling under the same roof, either because you no longer live with either of your parents, or because you already make enough money in your job to be able to move out and spare yourself the need to get used to living with total strangers who suddenly became step relatives (not because you dislike them, but because you're TOO OLD for that).

In this case, the mc discovered the guy he had sex with is the son of the same man his mother was dating and now wants to marry. Both the mc and his mother were each carrying out their own love-lives separate from each other and dating people without asking permission nor even introducing their partners to each other, so that is how this happened. Even before knowing the new stepdad's son was his ex lover, the mc was already planning to move out of his mother's house. Why? Because he's already a working adult who earns enough to live on his own, old enough to live alone and date without consulting his mother, and too old to be imposed the obligation of living with the strannger who's now going to get married with his mother. As an adult, the mc's merely asked to be polite to his mom's new guy, but he's not required to live with him nor call him father, let alone treat the man's son as a brother. The guy will be his step dad, but the guy's adult son will not be a brother, and will just be "his stepfather's son", no amount of marriages between their relatives is gonna suddenly turn them into siblings now. If on top of that, you add the fact that the son is someone he dated and slept with, then no amount of time as "step siblings" on paper is ever gonna erase that past. They *had* sex in the past, and people who've had sex will never see each other as family, even if a thousand relatives of theirs were to get married now.