Brother-Sister Incest: Legal (see here; although note the constraints on reproduction since, well, genetics). Also very, very rare, because it’s not like the Westermarck effect doesn’t still exist.
The Associated Worlds
…building civilizations with my space elves in space.
Interestingly, they could intentionally engineer the Westermarck effect out. Most Imperials would not, I would think, because the Westermarck is also it’s own meta. But someone will do it, and then have kids, and it will start to spread…
Actually, it occurs to me – and it really should have when I was writing this trope in the first place – that there’s at least one world where it would have been as a matter of reproductive policy: Valiár, where due to damage to the (very early-period) colony ship Swiftrunner only a very small number of colonists arrived there; resulting in a world populated by multiple clones of a greater in size – but still very small – set of first- and second-generation archetypes, all closely related, and all raised together. de-Westermarcking the population, given that natural reproduction was right out anyway given the size of their gene pool, would have been the better option compared to getting everyone to swig bromide in perpetuity…
Regarding spreading through the population, though, of this or of most other modifications, the thing to remember is that given two principal factors – first, that technology marches on, and most parents I am assuming will want their children to have genome v.235.2 rather than the natural recombination of their, say, v.227.0 and v.230.4; and second, that given cladization, the odds are good that a lot of couples can’t reproduce naturally even if they’re of a mind to, given both naturally occurring incompatibilities (a vacuum-native adapt and a subsea adapt have radically different organ systems) and fail-safe coding set up to avoid people ending up with inconvenient combinations of traits that don’t specifically clash but which don’t work well together. Between these issues and others of the same sort, virtually all of any late-period generation is designed, rather than natural, so any non-standard traits in there outside surface appearance and such are going to be there because the parents specifically requested them, rather than spreading automatically, which changes the propagation pattern quite a bit.