From reading the mangaka's other work, the first girl introduced typically doesn't end up with the MC. Yuhei and drummer girl feel so similar to Koyuki and Maho from Beck, I'd be surprised if the author didn't pair them before the manga's completion.
The author's formula is pretty straight-forward by now: good-natured sweet girl who is mutually attracted to MC but ultimately doesn't end up with him and the rough around the edges girl who ends up with MC. I'll still probably enjoy it.
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?