I went to a group party without telling my girlfriend in advance. Here I met a beautiful sister who didn't seem to care about me at all. A few days later, when I went to my girlfriend's house to play, I was surprised to find out that the beautiful sister at the party the other day was her sister! And unlike that day, today she deliberately approached me, seducing me with her charming lips. Unable to resist her beauty, we constantly secretly fucked each other whenever our girlfriend was absent. Some time later, I confessed my love to her, but she flatly refused. She only cares about other people's boyfriends, if I still belong to someone, our relationship can still be maintained...