Eromanga Sensei.

Masamune Izumi is a high school student who writes light novels. Sagiri Izumi is an antisocial girl who never leaves her room. A year ago, she became Masamune's stepsister. But one day, Masamune discovers a terrifying revelation: the artist “Eromanga-sensei,” who illustrates his novels, is none other than his sister Sagiri! His antisocial little sister, who lives under the same roof as him, uses a spicy pseudonym and draws obscenities?!