I was seduced by my boyfriend's promiscuous sister

I went to a group party without telling my girlfriend. There, I met a beautiful older sister who seemed uninterested in me. A few days later, when I visited my girlfriend's house, I was surprised to learn that the beautiful older sister from the party was her sister! Unlike that day, today she deliberately approached me, tempting me with her seductive lips. Unable to resist her beauty, we secretly had sex whenever my girlfriend was not around.

After some time, I confessed my feelings to her, but she bluntly rejected me. She only cared about someone else's boyfriend, and if I belonged to someone, our relationship could still be maintained...