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I am about to post a story but need some information to stay within canon, if I can. If no one can answer the question for me, I will have to review some of the episodes, which would be fun, but would take some time. The question is, by the time of the Romulan War, did Malcolm or Hoshi know that the Romulans were related to the Vulcans? Did they know they originated on Vulcan? Or did just Archer know, and perhaps T'Pol and Trip? This is not a really big deal but I might have to change my story a bit. Anyway, I will have to come up with a plausible reason why anyone in Star Fleet from Archer's time knew what the Romulans looked like when during TSO, long after the Romulan War, humans did not know.

For all I know, Spock was dumbfounded to see that the Romulans were similar to Vulcans.
So no one knew anything about them before.
My thought.

I assume that in ENT's time nobody knew what Romulans looked like. Otherwise Spock and Kirk wouldn't be so surprised to see a Romulan. Some Vulcans may have known, but kept it a deep secret.

In the Romulan war SF may have learn about Romulan language or technology, but not how Romulans looked like. People SF was fighting the Romulans in the war, I explain this in my stories by having them wearing a black mask on the face when interacting with other species, sort like the Breen. In one story I have the Romulans using chemicals just to wipe out all the DNA traces after a fight.

I can't remember any evidence during the Enterprise series that anyone learned who the Romulans were. Given that it was not widely known well after the war (however implausible that may seem) it would be easy to argue that the Romulans managed to keep their secret all that time.

On the other hand if it's helpful to your plot to have some people know then it would be easy to stay in canon by inventing a justification for it being kept a secret. It doesn't need to be a particularly plausible justification. In fact, for consistency with the rest of canon it might be better if it was obviously made up on the spur of the moment to justify some trivial plot point. But preventing a major diplomatic falling out between Earth and Vulcan due to Vulcan's secret duplicity being exposed seems to be a favorite theme which I can easily believe.

In TOS, the explanation seemed to be that the Romulan War was fought ship to ship and there was no personal contact. From ENT some Vu!cans (e.g. V'Las) knew and anyone else is open to speculation. If Malcolm and Hoshi learned somehow there would need to be a reason for them to keep it secret.

Just a tentative suggestion - could Malcolm hear about it via his Section 31 connections? then he would HAVE to keep it a secret.

I've always thought it highly implausible for no one to know about who the Romulans are. The Starfleet rank and file probably don't know (as witnessed in Balance of Terror) but I think some of the leaders on Earth - and later the Federation - does know. Obviously Section 31 knows, and thus maybe Reed. But the Vulcan-Romulan connection is in all likelihood kept secret as to not embarrass the Vulcan allies, both during the Romulan War and then to keep the newly formed UFP intact.

All this has its own logic, but building a history of knowledge of what the Romulans are on the part of some, though few, honestly removes from them charm and puissance.