18+ Monobeno: Sonogo no Natsuha no Nanatsu no Natsu

Sawai Tooru and his younger sister Natsuha venture back to their rural home town Monobeno – their first visit in six years. Here they reunite with their childhood friend Alice (a girl who labels herself as Tooru's fiancee), and a Yamori Youkai in the form of a small child.

After their nostalgic reunion it was not long before the town readies itself for the night of Himemiya Ryuu Shichi Seisai. During the festival, the Monobeno's ancient, 'Face Dance' was being performed when Natsuha suddenly collapsed and was rushed home. The next night, Natsuha's body turned into that of an adult's and her lower body bled for the first time. Tooru, fearing for his imouto's life hastened in his search of a cure...

(Source: VNDB)