The Elder Sister-Like One 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...