They've gotta be cousins and aunts/uncles or something. Ninne in Camp Dragonhead seems to be a noble of Fortemps, for example. 1000 years of tracking your pedigree means they'd be keeping tabs on a lot of family branches.

IRL for example they managed to find a relative or two of King Richard III to confirm via DNA that the body found in the parking lot was Richard...so quite possibly anyone up to a certain degree of separation would be considered a noble of a high house, with anyone going outside of those lines being 'demoted' to a different noble house. After all, everything we see is condensed...so we need to multiply the number of people we see in any town or area by about 10 (except for some adventurers, I think) to have a good grasp of how many people we haven't met, or how many are populating the city-states.

I think it would be funny to assume it's Artoirel with the fish collection, though. XD