The Elder Sister-Like One

The Elder Sister-like One (姉なるもの, Ane Naru Mono) is a manga by Pochi Iida. It began its serialization in Dengeki G's Comic magazine in March 2016. Yen Press has licensed it for English release.

Plot
When an orphan encounters the eldritch demon Shub-Niggurath, the "Black Goat of the Woods with a Thousand Young" offers him one wish. His request: "Become my big sister."