姉なるもの / Ane naru mono Series by Iida Pochi.

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."

Yuu is now living with his new big sister-an inhuman being he encountered in his uncle's basement! It's the middle of summer, and the two of them are enjoying their days, playing in the garden, splitting watermelons, and taking naps together. Life as a family is simple yet pleasant for these unlikely siblings. Little by little, their bond seems to grow stronger, but Yuu has got an uneasy feeling he just can't shake...

Yuu's bond with his new older sister deepens in the latest volume of this Cthulhu-inspired tale!

夕の「お願い」により破滅を免れた謎の少女・陽（ハル）。足繁く蓮沼家を訪れるようになり、夕と千夜を翻弄して楽しむ彼女の姿は、あたかも新しく増えたもう一人の姉のようであった。しかし千夜は内心穏やかでなく――。