Well, hypotheticals aside, if Addison and Christian have a sexual relationship, then as uncle and niece, it is incest. That's the easy part. In terms of legality (just as a distinction, since legality isn't being discussed here), that would then come down to where in the world (or where in the country) you are, as the laws differ. For tags here, I think it would count based on the "it's incest" part, regardless of how far removed the two relatives are. They are still relatives. However, tags only cover what is actually in the game, and I don't think a game would be tagged until it is "on camera", so to speak, though that would depend on the mods (as an example, would a game have the "anal sex" tag based on two characters saying that they had anal sex last night, although it wasn't shown? It wouldn't get tagged until you actually have the anal sex scene, and I would say it's the same here).
That's the other prominent theory, both for who it could be and what it could mean.