This is what had me re-reading the chapter. I thought maybe I misinterpreted, because I absolutely thought this was going an onee-sama ship route. I woulf have sooner thought Elis and Carla were fighting for who gets to date Onee-sama, and that they weren't into each other in that regard. But it seems like, no, little sister x little sister is what's happening?
I'm torn on the desire for flashbacks. Like another commenter, I appreciate the pacing and the progess. It's lovely to see the characters in a new stage. But with stuff like this development/reveal, I'm really left wanting some explanations or context to how we got here. I swear Elis only ever seemed interested in her onee-sama. And carla might've had a begrudging respect for elis at most. Now i wonder if i totally failed comprehension in earlier chapters