Was it the games, where you could only catch one of each? But then the pokedex entries explain that they form small groups, so...?
Anyway, yeah, I would be a bit uncomfortable writing twincest. As I just showed, though, I could do a story about two Latis that aren't related. Would that be okay, or did you pick the Latis specifically because you wanted twincest to be present?
In any case, incest is one of my favorite themes. Plays a part in most of my work, and I've written a few essays on the subject. Maybe I can be tossed that prompt, or at least something anyone feels fitting?