I seduced my girlfriend's younger sister and had sex with her

I've been dating my girlfriend for a while now and her younger sister has always been a huge turn on for me. One day, when my girlfriend was out, I decided to make my move. I went over to her house and sneaked in through the back door. I could hear her in the bathroom, so I knew she was alone. I went up to her room and found her laying on her bed, wearing tight boots and a shirt that showed off her belly. I couldn't resist any longer and I went over to her, starting to kiss her neck and undressing her. Eventually, we ended up on the floor, where I ate her out before fucking her hard from behind. It was the best sex I've ever had and I know my girlfriend would be furious if she found out.