The theory that Lila is Tomoe's daughter and Kagami's sister makes absolutely no sense, even though the show is making excuses for it. First, I'll list the evidence that the theory is true, and then I'll explain why none of it makes sense.

1) In "Oni-Chan" in Season 3 , Lila says she lived in Japan for a few years, says she looks like her mother, and looks at Kagami's picture angrily, demonstrating that she knows her or has some anger/envy towards her.

2) In "Perfection," she said that Kagami is like a sister to her.

3) In "Vampigami," she discovered that Tomoe helps Lila by offering technological equipment to aid in her plans, even having a personalized Tatsu just for her.

4) In "Yaksi Gozen," she discovered that this partnership between them has existed at least since the end of the fifth season. That's without mentioning that Lila calls Tomoe "mom" twice and says that Kagami is her favorite daughter.

5) In "The Dirtifiers," before Tomoe uses the Peacock Miraculous to create Kagami, we can see that she had a very similar hear to the first wig Lila wore in the first 5 seasons. At the end of the episode, Chrysalis calls Kagami "sister" once again and offers to train with swords in her place, implying that she has experience doing so.

Now, why doesn't it make sense?

1) Lila (Cerise) has European/Italian features (we don't know her true nationality tho). Kagami and Tomoe are Japanese with strong Asian features. The only logical explanation would be that Lila is a half-sister from another father.

2) If Lila is part of the Tsurugi Family, then what happened to make her distant from Kagami? Why didn't she live with her supposed mother and sister? Why would Tomoe have prevented and hidden Kagami from her own sister? And how and why would Lila have 3 mothers and different identities when her supposed real mother is right there?

3) The chronology of Kagami's birth will be a mess considering that Lila is really just a teenager (we don't know Lila's real age) and is the same age as Kagami.

4) Considering that Lila loves creating false emotional bonds and the fact that she has 3 different mothers makes me believe that she is being ironic when she says that Tomoe and Kagami are family, especially since considering Lila's deceitful and chaotic way of being wouldn't be well received in the eyes of Tomoe, who is obsessed with perfection and control. Lila is the complete opposite of the daughter Tomoe would like to have.

Lila being Tomoe's daugther is absolutely nonsense