Seikatsu Shuukan The Animation

Shuntarou is an average high school porn addict. His sexual desires are directed toward his younger sisters, who he deeply adores in a way no older brother should. He even resorts to stealing their underwear and satisfying himself to it, for which they deeply despise him. His only wish is to have a caring and kind older sister who would be the complete opposite of his mean siblings. After one such session with young women's panties, Shuntarou spots a mysterious new video game on his computer. Although he has no recollection of ever downloading it, the title "Older Sister Week-Long Experience" sounds too alluring to pass on. While the game seemingly does not start, the next day he finds himself shrunken to the size of an 11-year-old.