Sayuri-San no Imouto wa Tenshi小百合さんの妹は天使; An Angel Who Had Swooped Down Was My Little Sister; Sayuri’s Little Sister Is An Angel

Sayuri's parents divorced when she was young, her father took custody of her little sister, Mikoto, and she lived with her mother. Not having seen her little sister in thirteen years, Sayuri visits the park they played at as children and finds Mikoto there. Mikoto is a pretty girl, but why does she have wings?