Best Sister Ever

I graduated high school back in May. As a graduation present me and my best friend decided to drive to the beach for a week. It was a really long drive but I had a plan. My big sister (age 23) lives in North Carolina, about mid way for our drive. My plan was to stop in on her and crash overnight. Of course I didn't call ahead. When I got there she wasn't home. I called her and she pointed me to her hide a key. As we were hanging out waiting on her to come home I noticed several pieces of open mail sitting out on the counter. One of them was a pay stub. It was a funny name that sounded nothing like the dental office where she was supposed to be working. I googled the name and got a major surprise. It was an escort service! Now my sister is very pretty, great body, but she always tried to be the good , mature kid.
When she got home she realized what was on the counter and out of the corner of my eye I saw her ditch it into a drawer and out of sight. Later I couldn't help it, I had to ask. She freaked out. After I calmed her down I had to nearly promise in blood I would never tell our parents.
The next morning me and my buddy were preparing to leave when my sister stopped us. She asked us to stick around for a couple of hours and gave a sneaky grin when she said "you won't regret it". Just a few minutes later the doorbell rang. My sister answered the door and in walked a super hot girl about my sisters age. She introduced her as a friend from work. My sister grabbed my friends hand and led him into her bedroom. Her friend gave me a kiss and said she was for me. For the next couple of hours the friend fucked me in every was possible! And I knew, from the sounds coming out of the bedroom, my sister was making my buddy very happy.
When they finally came out of the bedroom my sister gave me a hug, kiss and thanked me for my promise. During the rest of our drive we compared notes. What he told me about my sister had me wishing we could have traded off!