Is there a term for the opposite of the Chuck Cunningham Syndrome?

Yes, there is a term and it is known as "Reverse" Chuck Cunningham Syndrome"

A reverse Cunningham is essentially the opposite of Chuck Cunningham syndrome, in which a previously-unknown and un-alluded-to character is introduced under the context that they had always been there, and had significantly interacted with the other characters before. This causes even more continuity problems than regular Cunningham Syndrome, as although it's easy to overlook the sudden absence of a character, it's harder to ignore the sudden appearance of a character, especially one that we are expected to believe was part of the regular cast, all along. Perhaps because it creates even more credibility problems, producers are much less likely to suddenly introduce characters whose presence should have been at least alluded to or explained earlier. But it has happened on a few occasions.

A handful of examples is also given there at the end of the article.

I know this is an old question, but for what it's worth, TV Tropes (warning! major time sink) gives this the much shorter name of "Remember The New Guy". They describe it as:

A previously unmentioned character who suddenly appears without notice and who we are told was really there all along but just... off screen or something. It comes about because the writers want to introduce a new character, but don't want to come up with a complex introduction where everyone meets them and learns what they're like- it's just plain easier (and lazier) to pretend that everyone automatically knows them, and the relationship is ready-made. "Oh hey, that's my old rival!" "Oh, this is my brother/sister whom I've never told you about," etc.

In other words, this is a relative or a friend or even a rival whom we really should have met, or at least heard about before, since the other characters would have us believe they have been crewing the same starship, working on the same project or sitting on the other side of the same classroom for years.

Sondra Huxtable is indeed listed as an example of this trope, while Chuck Cunningham Syndrome is mentioned as being the opposite.