Imouto ni Kiite Minai to

Daichi Fukumori, just starting his second year of high school, has been lying about having a younger sister in order to have something in common with the girl he had a crush on. When Daichi finally decided to tell the truth, a mysterious girl suddenly appeared and started calling him "big brother"! Daichi was surprised to see this sister, who turned out not to be a lie, but what awaited him was an even more outrageous situation! Hey, hey, I better have a talk with my......... sister before things get out of hand!

(Source: Monthly Ace, translated)