My lover's beautiful sister always invites me to have sex

I am living with my girlfriend as husband and wife because we are deciding to get married. One day her older sister Yuko Ono came to visit and was going to stay for a few days. Yuko is an open-minded and comfortable person, so she often dresses revealingly, which attracts my attention. And then just one day we had feelings for each other. The next day, my girlfriend went on a business trip for three days. And truly this will be a time in paradise for me. I will fuck Yuko's sister until I have no strength left.