I'm wondering... did Rinna have a chance from the beginning? since earlier chapters, the series screamed Sayo. Even every moment happened lead Mei to think about Sayo's feeling. Of course, Rinna made Mei flattered here and there, but compared to Sayo, their relationship was so underdeveloped I didn't even think she was a rival to Sayo. Somehow, it feels like Rinna was just there for a plot device of love triangle with no chance to win.
Also, I don't think poly route works since Sayo clearly didn't want to share her love (hence there was no arrow grow from Sayo to Rinna ever. she even got jealous when Mei and Rinna were cleaning the bath together). People started to expect poly ending only when there was a poly chapter. Even that chapter ended with a small and frail arrow from Rinna to Sayo, then never developed after.