Elder Sister-Like One, The vol. 3 (manga)

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 …”