Stepsister's workout routine arouses me

I've always been a bit of a perv when it comes to my stepsisters. Don't ask me why, it's just how I am. Every time they're working out, I can't help but get a little excited. I know it's wrong, but I can't help it. The other day, I was spying on them again, and it got the best of me. I snuck up behind one of them and gave her a little surprise. She was shocked at first, but then she got into it. We started making out and things quickly escalated. Before I knew it, we were both naked and getting down and dirty. The other sister joined in, and before long, we had a full-blown threesome going on. It was the hottest thing I've ever experienced. And the best part? It never happened.