Wow, I was wondering why there was an explosion of comments, but this was the reason? Did you folks not read the synopsis before deciding to pick up this story?
Oh well. Might as well give my two cents, but if there were that many people reading the manga, you guys should comment more. Let's have some good discussion as it comes up.
So the incest...the way I see it, Abel did it in a rather clumsy and stupidly ineffective way, but he isn't really "wrong" per se for going the way he did. There's not enough communication, but he also doesn't really believe anyone would listen because...well, look at his stubborn father. I would certainly consider his desire to avoid his sister being too overdependent on him a sign of strong brotherly love-not to mention, I'm personally kinda into a Firo ship.
I still think it's stupid that his parents never bothered to tell him something so fundamental to his future in his whole life until it came up. They kinda just sprang this on him and his stupid father decided to rush things when he realized his son was feeling uncomfortable with the idea. It's one thing to have your own culture, it's another to not teach your kid enough of it and suddenly force it when they're not ready.
Practically, there doesn't seem to be any real negative genetic repercussions to sibling marriages in this village, so I don't think we should worry on that front. If Abel decided to just go with it, there wouldn't be any problems.
Morally, I think it's half-and-half. This is a different world, and it's not a harmful practice, so there's no issues with doing it. It's their culture. But Abel is clearly against it (though he could've been more explicit with communicating this and exploring other avenues before just bailing) and I do take moral issue with how he was almost forced into the union. It's a taboo to him as an otherworlder, and as long as he doesn't want it, forcing him would be immoral.