Amateur stepsister secretly watches me and we end up having sex

I was casually hanging out with my step-sis when she started to act a little strange. She kept staring at me and then quickly looked away, making me feel a bit uncomfortable. I didn't know what to make of it, but I decided to confront her about it. As it turns out, she had been secretly watching us having sex and it had been driving her crazy. I couldn't believe it, but I was intrigued by the thought of her joining in. So, after a few more drinks, we ended up having sex together. It was a wild ride, with her showing me just how much she wanted to be a part of our sexual encounters. It was a hot and steamy experience, with her revealing her amazing big tits and giving me an unforgettable blowjob. It's not every day you get to have a threesome with your step-sis and your girlfriend, but we made it happen.