Fall in love with your lover's younger sister

I and my girlfriend have been living together in Tokyo for four years. She never thought about getting married, always working night shifts and coming home in the morning, so for the past three months, I haven't been intimate with her. With the university entrance exam approaching, Hikari, my girlfriend's younger sister, came to Tokyo to take the exam and stayed at my house for a day. She is really beautiful, and she wasn't wearing a bra, so when she looked at my private part, it automatically got aroused. While both of us were in the bathroom, suddenly there was an earthquake, and she and I were close to each other, causing her to notice that my private part was pointing straight at her.

And from that moment on, she started to seduce me, making me ejaculate inside her continuously. I knew she liked me, every time I talked about buying a house with my girlfriend, she would get upset. When she left, I realized that I wanted to be with her more, but she knew we shouldn't be together, so she left. A few months later, my girlfriend and I got married, she also got into the university she desired, she wished me happiness, the days with me were the best time of her life...