The Elder Sister-like One is the English translation of Iida Pochi’s Ane Naru Mono.
Security, happiness, a loving family: Yuu has had none of these since a car crash left him an orphan at age five. Passed from one resentful relative to another, Yuu has finally landed in his uncle’s household.
The uncle is too obsessed with Dark Knowledge Man Was Not Meant to Know to care much about Yuu. However. Yuu’s uncle does provide him with shelter, clothes, and food. This benign neglect is more than Yuu’s other relatives gave him.
When his uncle descends into madness, Yuu must find the uncle’s health card in order to call in the health services. Failing to find it in the house, he ventures into the storehouse, the one part of the household his uncle had declared off limits. There the unprepared boy finds himself face to face with the sanity-shattering reality of Shub-Niggurath, the Black Goat of the Woods of a Thousand Young, who appears, possibly for marketing reasons, as an extremely buxom, scantily-clad, be-tentacled, obviously demonic woman.
Being somewhat naive, Yuu jumps to the conclusion that Shub-Niggurath must be an angel.