I'm a college student living with my brother, and I have an extreme sister complex, to the point where I can only get excited by sister-themed stuff. One day, Hanami-san, who is about to marry my brother, moves in with us. She's even more beautiful and has a better figure than I imagined, making me flustered. Then she smiles and says, 'It feels like I've gained a little brother! Think of me as your big sister!'—Wait, what? Sister? So, if she's marrying my brother, that makes her my sister! The 'sister' I've dreamed about is finally here, and I can't contain my excitement. However, since she's my brother's girlfriend, I try to restrain myself. But Hanami-san starts stretching after a bath, shamelessly showing off her massive ass, and even asks me for help. There's no way I can hold back when faced with my sister's overly erotic giant butt…
