Caught Red-Handed

I snuck into my fiancé's apartment while he was away for work. I hadn't seen anyone naked but him in years and it was driving me crazy, so I started playing with myself. Suddenly, there was a knock at the door, it was his sister. She walked in, saw everything, and didn't even flinch. We ended up having sex right then and there on the couch, her knowing full well it would drive my fiancé insane if he ever found out. Now I'm scared to death what happens next. What do you think I should do? Am I a monster for wanting more?