Thank you for the ask, am glad to hear it!
I will explain one other concrete detail that I feel is important:
It's a college AU, but mostly because it's a good excuse for Dia to be alone and at a low point: moving away to go to college, separated from Aqours, and this pressure to excel in school in an unfamiliar place + from her family (except Ruby) to still come back and lead the estate leads her to kinda spiral into being a mess + gains, so she distances herself from everyone as a result

To answer the Riko question: at first she feels super conflicted on Dia's gain for multiple reasons, one of which I absolutely believe she's into feeder/feeder doujins, but since they haven't properly talked about their relationship, she feels like that she is taking advantage of Dia, while Dia feels the opposite and feels like she is taking advantage of Riko's kind and caring nature, so things get awkward for a while 😊

As for with Ruby.... Definitely Ruby is one of the first to try to reach out to Dia during her low point, but sadly isn't able to help her sis like Riko could, so she would beat herself up over not being there for her sis again

I do plan for the sisters to properly reconcile, but for a while it's also a little strained cuz Dia feels awful for "abandoning" Ruby, but is still gathering the courage to properly talk to her about everything, while Ruby completely understands why Dia would run, she can't help but feel a little frustrated at being thrusted into this role, + worries that she would end up blowing up at her sis when they meet in person again

Sorry for the long dump, but once again thank you for the ask!