Lapis being secretly the little sister (Adopted commoner little sis/ Aurora) would be an amusing idea....But that would play into question....Lapis show absolutely no signs of being able to predict the future though...It's a interesting an amusing 'THEORY/hypothesis' but to me seems a bit unlikely to me...It is 'possible' Theory. Wonder which person came up with the theory though since some people seem to be like this theory and think this theory is 'likely possible' instead of just a possibility.
Though that theory would only work assuming Lapis was actually the one with the future sight instead of the OP magic talent/potential....(The adopted sis was actually the OP powerful talented instead of future sight and Lapis actually weak in magic but had future sight...Magic potential seems something your born with and seems hard to train to increase even with intense training and effort so unless Aurora had both future sight and OP talent or just OP magic talent? Perhaps Lapis had future sight and OP magic potential as well?). Then again this so-called theory could be possible if MC was playing the Gacha game version showing that that Lapis (Real Lapis survive) and that isekai world she was transported into was the world version where Lapis (Aurora survive but pretend to be Lapis).
This is also assuming the father/ Lapis father would even agree to the fking idea of his adopted daughter pretending to be his real blood daughter to avoid the Anti-nobles feeling a sense of victory for their assassination attempt or etc.
PLUS+++....You also have to remember cat boy capture target mind reading ability here...Cat boy from chapter 9 mind reading ability might not be a perfect 100% accurately read everything in your brain...but he can scan and see a lot of images and memories, he saw a lot of Gacha event images from MC's event and he was a bit confused what they meant. Either Lapis (Aurora) was a very skilled actor or Cat boy's mind reading powers is not that good (Can only see into surface memories of few weeks or what they are thinking right now).
[LIST OF DETAILS AND FACTORS TO CONSIDER from various chapters (You have to factor in the butterfly effect of MC existence and actions subtly changing the story here.)]
That would be Truly ironically...That the adopted commoner little sister hates commoners so much that she wants to kill every single commoner in the Anti-noble fraction despite being a commoner herself (Chapter 2- Lapis comment about the commoner being barbaric savages and asking MC if she was a former noble or something as Lapis has a minor flash back to herself + Lil sis///Also that fking innocent kind smile lol...Lapis good actor that fooled the entire player base of the gacha game).
The Gacha game where Lapis tricked all the players into thinking she was a kind hearted girl to the [PLAYER character] but was actually the villainess/final boss of the game.
Chapter 1 events changed slightly due to MC being summon instead of Lapis' normal summon creature (Though I wonder what exactly was Lapis' original normal summon creature instead of the MC....In chapter 2 it shows seem to be a strong demon black dragon?). Lapis stopping the fire burst out/having time to notice the warning signs from the protagonist due to being finished with her summon her 3 OP dragons and people not making a fuss/trouble about MC being summon. There is a possible sentence/comment in chapter 1 that might point/aid in the Lapis is actually Aurora 'Theory' but it could also be a false flag and totally just something said due to pride. Point and digital cookies to people who might remember which page lol.
Chapter 3 (Event changed, after familiar battle, Lapis praised Diana as your power is wonderful but thanks to MC action
Lapis never got to say those words and Diana realizing her magic power could hurt other humans/people, start of Diana's crush and affection towards MC start here.)
Chapter 4 (Meeting with prince, end of Gacha game chapter 1 prologue [20 min play time final scene of chapter 1] but it took about 1 month to get to this scene IRL for MC after isekai./// CHAPTER 4- lapis comment to MC is giving strong vibes that she is the original Lapis...But that could also be a fail flag too and she might actually be Aurora in reality...Seem unlikely but anything is possible....Unlikely theory [To me and other people] but possible. Then that fking comment again I see...Could be pride, coping mechanism like "I am batman" or could be sign that theory is very possible? Anything is possible. )
Chapter 8 (Prince and Lapis first meeting, Lapis here seem to have same hairstyle and height as shown in flashback with her little sis...So unless little sister had a shown growth spur that is one point against the Aurora is pretending to be Lapis' theory....Though that could also be misleading and misremembering from Prince's memories....And basically fking lying to the prince and member of royalty with Lapis' father consent...Prince shows no signs of knowing that Lapis is actually Aurora theory though?)
Chapter 9. VINO!!! Lapis' spy that is gathering information from the commners and nobles for Lapis....Vino is very disgusted and afraid of Lapis and despite the possible multiple times he likely read Lapis memories...Doesn't seem to be any signs that Vino knows about Lapis possibly being Aurora...Just that Lapis intends to kill a lot of Commoners in the Anti-noble fraction... (Not just kill some to protect her family and people amount....Full blown Annhilation type that scares him which is understandable for revenge...)
Chapter 10 interaction with Lapis father and her...Dammit kind of Sus and point for the Lapis might be Aurora theory though I admit....but could also be just tense strain relationship where Lapis drifted apart due to the death of Aurora/ adopted little sis...
Rebeus is a Sh*t-head, but eh whatever a useful pawn for Lapis. He thought he is so clever and smart but eh he is just dancing in Lapis' palm without knowing when he tried to manipulate Lapis (Chapter 15-16).
Chapter 17 (Hmmm...Possible clue or just false flag, not enough information from that minor flashback scene. Not detail enough image/picture from that flashback.)
Like I said...The theory is 'possible (More than 10% but less then 30% chance max in my opinion)' but the chances of Lapis (Lapis is just actually the 'real Lapis') is also true since Revenge for her adopted little sister's death is a valid and strong motivator for revenge as well.