I'm still not completely buying it, but this does leave room for more ambiguity than I initially assumed.
Edit: I am completely on board with the big sister route. This just seems like an clunky opening that wasn't needed.
At this point, I don't think the author will fake out the yuri tease a 2nd time. Then again, she seems to really planting these instances of conflict just to reveal them as misunderstandings later.
But the bedridden character whom Sora speaks to and falls for in chapter 20 is quite clearly Miya. Unless this is a retcon, it just can't be her big sister.
Though she probably wanted to, I don't think Sayaka offed him. He's the only lead she has in regards to Kurumi's whereabouts. Maybe she gave a non-lethal shot and pressed him again.
Also how much does Japan hate Russia to make the med student both evilly manipulative and painfully oblivious?
This is the impression I got, that she's backhandedly encouraging him to act.
Like she's saying, "look, unless you ask me out, nothing is happening between us"
Senpai could have made a formula to age her up. She did the exact opposite not too long ago, and how different can it be from the one that ages herself up? But that would be too easy, gotta make it convoluted with an even bigger age gap