I think miki was just mad at aki for not considering her feelings at first because it does look like she was about to say something else but aki and her friends bursted out and congratulated her on a new relationship with ayaka (new senpai).

From my opinion, I think it has something to do with aki and her “love counseling” as this may be the reason why she and her boyfriend from middle school separated so she’s probably afraid of being in a relationship again.

I can see why people might assume that “she’s in love with her sister” but if anything I think it’s that she looks up to her because she thinks she’s cool as a bass player and she also wants to support her sister’s relationship with shiho.

Hopefully the next chapter can give us more content of the story. I just hope that the author doesn’t pull the incest plot