Lol, I get where you're coming from but I actually understood it pretty well. Just imagine if the seme was a girl instead? It would be super creepy if the brother of a woman who suddenly becomes step mother for a grown girl, suddenly developed feelings and slept with his step-niece. Twenty years old is still just young enough to feel iffy, specially since the "girl" in question has never been in a relationship before. We don't feel is such a big deal because the "girl" in this case is actually the seme/aggressor in the relationship, but to anyone looking outside in, it would be super uncomfortable. Plus we all know Asian countries aren't super accepting of homosexuality, so it becomes a double whammy. I actually had no problem with his reticence and completely understood it. It would've felt wrong if he had just jumped into it, to be honest.
Glad they live happily now but i gotta say i'm a little bit irritated at the uke like they're feeling are obviously mutual but he run away because he is afraid? Only to go back to him in the end so like why didi he bother run away in the first place. And he also keep using uncle nephew as an excuse when they're not even blood related and even if they weren't uncle and nephew they still wouldn't be able to get married anyway so what's the big deal?