I did a reread of the available chapters, and I noticed something interesting. Outside of the first chapter, Yvonne only ever refers to Elsa by her last name, Dorothy. She calls her Elsa in her thoughts, but when addressing her by name it’s always Dorothy. It makes sense in the recent flashback as they had only met recently, but she keeps it up even after her exile.
On the other hand, Elsa has only ever referred to Yvonne formally 1&1/2 times, also in the first chapter. Even in the recent flashback she calls her “Miss Yvonne,” which is surprisingly informal given their respective statuses.