beta
I feel like there are at least a couple of obvious routes but here's one:
ML has liked MC from the start, but MC, being an Alpha, set boundaries and completely missed ML’s feelings for him. Yuuma, who’s MC’s twin, also liked ML but knew he didn’t stand a chance. Then, during one of Yuuma’s heat cycles, he lost control and ended up sleeping with ML. ML, affected by Yuuma’s pheromones, couldn’t hold back either—the fact that Yuuma looks exactly like MC didn't help
Feeling guilty about the situation and knowing that ML actually likes MC, Yuuma decides to leave. He figures an Alpha would be better suited to take care of his son anyway, so he leaves Sana.
I like to think Yuuma wasn’t a bad person to begin with ^^