姉なるもの / Ane naru mono Series by Iida Pochi., 飯田 ぽち。

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 pleasant life with his big sister continues, and slowly but surely, their bond is deepening. She's always there for him, whether they're at home, at the hospital, or...in the bath?! Yuu even gets to see an entirely new side of Chiyo when he comes home to find her drunk...But paradise can't last forever. When an unknown demonic creature suddenly makes an appearance, Chiyo's countenance darkens, and her true nature surfaces...

Haru has been taken captive by Chiyo, and her only hope of escape lies with Yuu and his desperate pleas on her behalf! His persistence pays off, as Chiyo eventually relents, but when Haru decides to stick around, Chiyo's none too pleased with her constant presence in the house. With two demonic "big sisters" to take care of him now, Yuu may have bitten off more than he can chew!