I mentioned it before, but these really are a dying people. Yuuri is already 10, but he hasn't got a single sibling, by the looks of it. In our history, he absolutely would have at the very least two if he was the oldest child. If he wasn't the oldest, there would naturally be more. The parents should still be producing kids, though many would die before adulthood, like children used to do back in the day. Of course there were also physiological/genetic exceptions, resulting in only one child despite a lot of trying. However, there don't seem to be a lot of children in general in this series. Siam is an only child as well. Even if they weren't attacked by a murderous enemy, they would go extinct anyway. Very little children and undeveloped medicine is a deadly combination. Add to that how the capital seemed to be a fairly large city, so they could lose what little population they have in one fell swoop if plague happened there.