Story
Mitsuko is a third year at an art high school. In the hallway, she found a painting which had won the school competition and would be hung up for a whole year; a nude painting of herself done by her little sister, even though Mitsuko had never posed for her. About sibling who draw naked drawing of each other.
Synopsis via AniList (English).
Preview
From the anime adaptation: Tatsuki Fujimoto 17-26.