Boku wa Onee-sama no Cosplay Doll

Protagonist Akira was a serious student, while his elder sister Yuuri put her otaku cosplay above all else.
One day Yuuri discovered something she could use against Akira, and it was his undoing. She coerced him into dressing up in one of her old outfits and took photos of him having sex.
From that day on she had complete power over him, and dressed him in all sorts of erotic outfits...
Their sweetly perverse days had begun.