@InvinCybul I think the last sister will be Ichika. She's known for a long time Miku's feelings, and hasn't done anything about it; so i believe in her chapter she's going to "confront" Miku, and "beat" her, passing with more score (that would stop Miku's goal, because I don't think a confession will occur at the moment).
P.D: Sorry about my english, I'm not very used to writing it.