Search Results: sleeping with the same sex

Last year before my girlfriend and I were exclusive and were free to see other people, she left town to spend Nee Years in Las Vegas with her friends. I was already a long term friend of her and her family and they were all secretly rooting for the two of us to get together. While she was gone her sister and I were invited to the same party. I didn't pick her up or make plans to spend time with her there but as the evening went on we both drank too much and stayed around each other for most of the night.
I went to leave and she went with me, saying she didn't want to be at the party alone. Instead of taking a cab to her apartment she hopped in my cab and went home with me. We had sex as soon as we got to my apartment, and she stayed the night. It was actually really great sex, too! The next morning we both wake up with hangovers plus she feels rotten for sleeping with me, telling me she knows how much her sister likes me and starts crying. We make a promise to never tell my girlfriend.
But now girlfriend wants to move in with me which is big step in our relationship. Should I finally open up about this? I've actually had nightmares where her sister tells her we hooked up and it really bothers me to this day.