Natsumi’s Secret Milk, also known as なつみのひみつのみるく, is a captivating Japanese web novel that delves into the intriguing relationship between two siblings. The story revolves around the protagonist's little sister, who introduces him to her unique concoction called 'little sister's milk', leading to the development of an unconventional bond between the siblings. Filled with adult themes and ecchi elements, this novel explores taboo subjects such as breast fetish, incest, and the dynamics of modern-day relationships.