SSIS-309 Fucking my girlfriend's sister for 3 days and 3 nights

Rika and I have been dating since high school. Maybe because they've been together for a long time, their feelings are no longer the same as before. Recently Rika got acquainted with a man in her senior year, who was superior to me in every way, making Rika feel shaken. That man was about to go on a trip and asked Rika to go with him, she agreed and took this opportunity to confirm our current situation. Even though Rika didn't say it, I still felt like something was going on. While sitting alone in a park near my house, Konan passed by. She knows everything, she sits and listens to my feelings, chats and helps me feel better. On the day Rika left for the trip, she waited for me outside the door and offered to replace Rika as my girlfriend for three days! And during these three days, I felt like I was back to the time when I first fell in love. I learned more about her and gradually fell in love with her. I tried to confess but she refused, I'm Rika's boyfriend, her younger sister, there will never be anything between me and her...