@bunny-k1m : iirc, the bit mentioned in this is that (not really a spoiler since it's already told in the story)
She's a Euclid child who had the Eredia ability instead of the Light ability Euclid use, so she was seen as worthless.
Then Dietrich's mother kidnapped her, and later on died protecting her from the assassin sent to kill off Anisa.
Since Dietrich's mom was pregnant at the same, people just kinda assume Anisa is the real daughter and raised her as Lagrange.
In the OG plot, Anisa would be revealed to be a Euclid by her real brother (the novel's male lead) and used to seal off Dietrich.