Heart Mark Oome OVA 1

Yuu Saigawara lives with his stepsister Reika, a shut-in erotic manga artist who uses him as a live reference for her explicit work. Whenever Reika finishes a manuscript or needs inspiration, she subjects Yuu to intense sexual encounters to ensure her art remains authentic. Despite his desire for a normal life and his friendship with a classmate, Yuu remains trapped as his sister's primary source of creative material.