My life as a new working adult has begun, and it's been a week since I started living with my girlfriend. Daily life is gradually becoming familiar, and I've been reasonably satisfied with how things are going. I feel happy, yet memories from my college days keep flashing through my mind. Back then, there were so many cute girls around me—compared to now, life feels a bit lonely. Is it really okay to grow up like this? Should I stay with my current girlfriend forever? Will we get married? Amid these thoughts, her younger sister came to visit our home. Her name is MINAMO. A female college student, she was exceptionally cute, especially in facial features, and gave off a delicate, petite impression. On top of that, she had big tits—her presence alone was captivating. Before I knew it, I was completely obsessed with my girlfriend's sister. The moment my girlfriend stepped away from the living room, MINAMO took my hand and pressed it against her breasts. "It's been a while since you've touched boobs other than my sister's, right?" Her words threw me into turmoil. ...No... No... No... Overwhelmed by that emotion, I ended up cheating, urged on by her. "Hey, wanna sneak around and do dirty things behind my sister's back?" And so, while being acutely aware of my girlfriend's presence nearby, our secret affair began. ...Heart pounding... Heart pounding... I was even more aroused than during regular sex. God, thank you so much!!!!!!!! Her younger sister, MINAMO, is absolutely the best!!!!!