From this chapter I think the dude now is just trying to get over his first crush while staying by their side. He's accepted he can't win and trying to move on. I really hope this is the case and the author don't pull a 180 and make him get together with the onee san
Dang, she just got thrown in and immediately got the situation.
I can see her and the MC getting together, if she does turn out to be the "girl that shows up later." I think as it stands, the impetus for her getting all crystal eyed at him is a little weak, but hey what triggers love doesn't have to be a big thing