Exciting new developments in the field of Sueology have allowed the widely-accepted theory that the more outstanding and canonically significant an active Mary Sue is, the less likely it becomes that the regulars they interact with could somehow never have heard of them before, to be proven beyond all reasonable doubt.

Mhari's Law of Sue Notoriety:

T x (SPa² + B) = R³.

As all research Sueologists know Mhari's Law, named after the redoubtable field researcher who first began to classify the various species of Sue that exist in the wild, states that the more amazing talents (T) an OC is burdened with, and the more overwhelmingly significant her past links (SPa) and blood ties (B) to one or more of the canon characters becomes, it becomes exponentially more likely that both the show's regulars and we the reading or viewing audience should already be able to recognize (R) her.

As the likelihood that the regular characters and we the readers should already know an OC increases, the probability that a character like this could actually exist in the fictional universe in question and still remain an enigma to the canon cast decreases, as Dr. Merlin's Constant states. No fan-created character could both loom unconvincingly large by the sheer magnitude of her condensed significance and specialness and still remain a mystery wrapped up in a riddle of a conundrum to the regular characters reluctantly sharing space with her.

( In which laila stops messing with equations )

Generally speaking, if the regular characters - not to mention the fandom at large - have never so much as suspected that [Character A] might have a long-lost younger sister, that's most likely because the character in question doesn't have a younger sister. If there's no reason to believe that [Team B] aren't every bit as unique as we always suspected they were, it's going to be very difficult for any fandom writer to claim that another character or group has been there for years, working for the same agency, doing the same job - only better. If [Group C] have never heard of the super-amazing foreign psychic girl who effortlessly outclasses the best student in the school and whose powers far surpass her tutors, it's not because the girl in question is charmingly modest but because there's nobody like that anywhere on campus.

If the regulars and fans haven't heard of someone very like your super-special OC already, the explanation for that oversight is probably very simple:

It's most likely that the OC doesn't exist.

It's up to the author if they want to make an OC intriguingly enigmatic or outstandingly remarkable. There's nothing wrong with wanting a new character to be either of them. What they cannot do, however, is try to suggest that any fan character - even an amazing Mary Sue - could somehow manage to be both specially special in the eyes of all comers and a mysterious mystery to all concerned without doing a Hell of a lot of legwork to explain why an apparent paradox isn't paradoxical at all.

I admit it: I know damn all about math or physics. I just thought this would be more amusing than Ye Olde Letter Format. Hell, it makes more sense than the science bit in skincare ads. Please don't hurt me, fanficrants.