Koushakuya no Jijo wa Sugata wo Kakusu

One morning, the second daughter of a marquis family—Celestina—vanishes without warning. The Second Prince who cares for her begins searching. But when arriving at the marquis’s estate, something strange has happened: everyone there seems to have forgotten the existence of the second daughter. It is as though she never existed. Amid this bizarre erasure, Celestina’s rare status as the holder of the “Windaria’s Snow-Moon-Flower”—a power that can affect the country’s fate—becomes clear. Why did she hide her identity? What secret mission lies in her heart? https://www.cmoa.jp/title/328826/ (Mangaupdates)