Their overall demeanour and Kandice saying "The fencing program is proving successful." might give the impression.
Gemma is well beyond 18 at this point, however, so keeping her a secret for all this time would've probably been extremely difficult.
Now, about Iniyah, things might be a little different. There is Kinda saying "I didn't even learn she [Gemma] had a sister until recently!" Iniyah wins the chess game in chapter 4 and says "Discovered check", which sounds..strange? Gemma replies "The game is yours, mother.", which could mean that Kandice wrote Iniyah's chess program. Gemma is also the only one that speaks during the fencing scene (only 1 sentence though). It could be that Gemma is training Iniyah, similarly to MC training Artemis.
Now, assuming they are androids (or even Iniyah only), why keep them a secret? Iirc they outshine anything TWR has produced so far and also Artemis for now. It would also take away from Artemis being this next big thing.
Gemma (non android) should have been a LI. ![Cautious :cautious: :cautious:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)