Nanami and her younger brother Yuzuru have lived together in a small house for ten years, sharing the same room. They sleep side by side on adjacent futons, and spending every day in the same space has become completely normal for them. Over time, Yuzuru has grown increasingly fascinated by his older sister's big tits. One night, he discovers Nanami secretly touching her pussy while he pretends to sleep. From that moment on, Nanami begins regularly giving Yuzuru handjobs, titty fucks, and full-on sex—treating him like her personal toy. Despite the strange dynamic, Yuzuru finds immense pleasure and satisfaction in their intimate relationship, which continues as an unspoken routine for years.