I would say she was more jealous and surprised in a cute-Sheryl way, she wasn't angry, just one of the girls in the triangle. Ok, I'm overly postive towards both girls, I thought Ranka was super cute in much the same way in epiosde 15.
There's a part later on where Ranka has to sing for her most precious person according to Miina and she thinks of Alto before singing. Miina realizes this and her own feelings towards Lion at this point.
I think some characters sing for different reasons in the story, Sheryl sang at the beginning as part of a concert and told Miina to sing with her later on.
Oh and Sheryl is totaly building her harem of slightly awkward, shy girls, Ranka found a strong rival in Miina. I mean, look:
This is the end screen when you beat the game for the first time, not a picture of Miina and Lion, but the true love story of the game.
Look at Sheryl's face and Miina and Ranka. I died.