Really, when siblings didn't grow up together, I barely think of it as incest at all. Except for the purposes of 'augh what am I doing' freakouts, because those are always fun. Er, from a writer's/reader's point of view. Probably not from the point of view of Nathan or Lucas.

They're only half-brothers (same father, different mothers), if that lessens the 'Eew!' factor at all.