Noa no Hakusei: Fushigi na Antique Shop

A small fortified city said to have been built in the shape of a heart. In the back alleys of this city, there is a mysterious antique store where "those who are truly seeking can find what they are looking for." The owner of the antique store is Riku, the elder sister, and Bon, the younger brother. This is the beginning of a beautiful and terrifying fantasy story about the interaction between the two, the customers who visit them, and the strange inhabitants of the town. (Source: Mangadex)