Natsumi's Secret Milk is a provocative web novel in Japanese that delves into the taboo relationship between two siblings as they navigate the complexities of their unusual bond. With themes of adult content and ecchi elements, this story explores a unique dynamic between a brother and his younger sister, marked by the intriguing concept of 'little sister's milk'. Fans of stories exploring controversial topics such as incest and fetish dynamics will find this novel fascinating and thought-provoking.