You know, if they're adult brothers who didn't grow up as family, that kills all of my actual ethical objections to incest (messing up the family dynamic, power issues that stem from being siblings, possible sexualization of the relationship from childhood, and the possibility of unhealthy children). There's still the 'Eew!' factor, but that's really a personal taste thing.

So, if you wanted to ship Lucas/Nathan, I wouldn't be that put off.