Sora-iro no Hibiki is a spin-off series of Papa no iukoto wo Kikinasai! (https://bato.to/comic/_/comics/papa-no-iukoto-wo-kikinasai-r960). This particular series is a prequel, starting about one year before the regular series, that tells the story of the Elder sister, Sora, and her new life as a junior high freshman.