Hell Is Dark with No Flowers, Vol. 3 (manga)

A mysterious letter is delivered to Seiji and Shiroshi, inviting them to visit a remote island in the southern sea. There, what awaits them is a beautiful living doll and a cursed family living in a mansion… But they aren’t the island’s only inhabitants. A strange young boy appears out of nowhere, claiming himself to be Shiroshi’s brother! What is his true identity…?