Due to Myne's display of her mana at the end of Part 2 Volume 2 during the trombe mission she becomes a very valuable target for nobles (due to the whole country suffering from a mana shortage) which makes it highly probably that somebody is going to try and kidnap here to syphon off her mana. Sadly the temple isn't well respected and is completely subservient to real nobility so despite being the High Priest Ferdinand can't really protect Myne in the long term so discussions begin between him and Karstedt to have the latter adopt her so that she's protected (another reason for the adoption is that because Myne has so much mana she really needs to be taught how to use magic so that it can be used for the sake of the duchy but only the children of nobility are allowed to do so). Later on in Part 2 Volume 3 when Ferdinand and Karstedt tell Myne their plan she initially refuses because she doesn't want to be separated from her family but Ferdinand insists because as she is she's a massive danger to people around her but says she can wait until she's ten (which is the age noble children go to the Royal Academy to learn magic). Later in the volume Ferdinand calls a meeting with Myne and her parents when he explains the situation including the risk of Myne being executed if she is judged to be a danger with her high level of mana and lack of training so her parents agree to her being adopted into nobility when she's ten. Next we meet a new blue robed priest called Sylvester who builds a relationship with Myne throughout the rest of the volume and just before he leaves he gives Myne a black necklace style magic tool saying it's a form of protection charm and that if she's ever in a bad spot (due to nobles) she should activate it with her blood and then he'll come running to save her.
Now we're into Part 2 Volume 4. We learn that carriage with a noble was let into the city of Ehrenfest despite the archduke clamping down really hard on nobles entering the city (in part due to Myne) and straight after somebody tries to kidnap Myne and Tuuli but due to Gunther and Damuel (the latter was assigned to be Myne's guard as part of his punishment from the trombe mission) they've saved. Myne realises how serious this situation is and pushes her blood (she got cut during the attempted kidnapping) onto the magic tool Sylvester gave her before going with Gunther and Damuel to the temple to seek help with Ferdinand. However when they arrive they don't find Ferdinand but instead the High Bishop and a noble called Count Bindewald (who comes from a duchy south of Ehrenfest called Ahrensbach). The High Bishop had conspired with his sister (the archduke's mother) to obtain a fake entry permit to get Bindewald into the city so that he could kidnap Myne. Naturally a fight occurs but things look grim until Ferdinand shows up however things look bad at first because Myne, a commoner, attacked a noble in self defence which is not allowed so Myne is quite likely be be executed for this along with her family and attendants, but once Ferdinand notices the activated magic tool from Sylvester things change. Ferdinand tells Myne that she needs to steel her resolve right away and get ready to be adopted immediately (a little over two years earlier than originally planned) and by Sylvester now instead of Karstedt, naturally Myne agrees to this and Ferdinand quickly detains the High Bishop and Bindewald. Next we have Sylvester and Karstedt show up with it being revealed that Sylvester isn't a blue robed priest but he's in fact the archduke of Ehrenfest (also know as Aub Ehrenfest) and that that magic tool he gave Myne was a tool for adoption so when Myne put her blood on it she became his adopted daughter, meaning that Myne wasn't a commoner attacking a noble and in fact those two were attack the adopted daughter of Aub Ehrenfest.
Following this it's clean up. It's decided that Ferdinand, Sylvester, and Karstedt are going to create a fake story for Myne to make her transition into nobility much easier and safer: the story is that Myne died here due to Bindewald and Myne now becomes Rozemyne - the fictional biological daughter of Karstedt and his (deceased) third wife Rozemary who had been hidden in the temple for political reasons, furthermore Rozemyne is a year younger than Myne which means that she has yet to have a baptism ceremony yet (this means she can get a new baptism as a noble as well as giving them a public event to introduced her to nobility), and that immediately following her baptism ceremony it'll be announced that Sylvester is adopting her due to her genius and absurd levels of mana. Finally, we have Myne saying farewell to her family and they all have to sign a magic contract stating that as long as they're in the Ehrenfest duchy they can never again meet as family - Gunther is no longer her dad but just a commoner city guard; Tuuli is no longer her sister, just a commoner seamstress; Effa is no longer her mother, just a commoner dyer; Kamil (who was born in Part 2 Volume 3) is no longer her brother, just an infant commoner - however they can still meet as long as it's a proper noble-commoner relationship.