[Spoilers] Ascendance of a Bookworm: Adopted Daughter of an Archduke Episode 1 Discussion

Okay, so they are basically retconning her to be born of Karstedt and his late 3rd wife so that she is of high enough status to be adopted by the Aub

sylvester, the archduke, is adopting her.

she's being treated as karstedt and his first wife elvira's daughter. the story before that (the one they cooked up at the end of season 3) was that she was karstedt and his third wife rozemary's daughter. myne had to be a noble to be adopted by the archduke and she needed to be adopted by the archduke to save her family.

she can't stay with karstedt's family because the adopted daughter of the archduke is too high in status to stay with archnobles

To add to the previous comments, because even though it's been years since I read the novels, I noticed they cut down A LOT.

This is from the wiki, btw:

Originally, Sylvester suggested that Rozemyne be baptized as the daughter of Karstedt's late third wife, Rozemary, who like Rozemyne had an unusually high amount of mana. According to this story, Rozemyne was hidden in the Ehrenfest Temple in secret to protect her from the power struggle between Karstedt's first two wives and his third wife after Rozemary’s death.

However, Elvira immediately saw through this story. She pointed out that introducing Rozemyne as Rozemary’s daughter would reignite the familial conflict. Instead, she agreed to baptize Rozemyne as her own true daughter as a favor to Ferdinand. In private, Cornelius is told the original cover story, that Rozemyne is Rozemary’s daughter.
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So, to keep things as simple as possible:

• Sylvester and Co need the nobles to believe that Rozemyne is a noble from birth (No noble would buy putting a stinky commoner as an adopted daughter of an Aub).
• To do this, they say that she's Karstedt's daughter from her third wife, who had a lot of mana. (Which is why they make Rozemyne bless the entire room in her baptism, since noble children usually bless on or two people in their baptism, blessing a room full of people is a huge display of mana.)
• With her being Karstedt's (A high noble) daughter, having copious amounts of mana, plus her talent and compassion at the temple, they have enough reasons to justify Sylvester adopting her.
• From now on, all the nobles believe she is Karstedt's biological daughter (Except Elvira, who saw right through it, but decided to ignore it because she fangirls for Ferdinand that much.)
• This is important later on, but there are two major factions of nobles beefing for power. Karstedt's third wife, Rozemary was from the opposite faction from his first two wives, which is why there was some family drama involved in "sending" Rozemyne to the temple.