It was a two part story i believe. The first part was a husband went with his wifes parents, in an rv or something, to help out the dad with some house work. The father in law was driving and he went to lay down in the back and the mother in law got tired to so she decided to go sleep in the back as well. well there was only 1 bed so they would have to share. As the ride went on they both started getting turn on because they were grinding on each other. The son in law decided to expose himself through the boxer to get a better feel cause he thought the mother in law was sleeping. After a few moments of grinding on her ass she gets turned on even more and gets up to go to the bathroom. The son in law thought he was caught and was getting scared. When the mother came back she just laid back down and grinded her ass on her son in law. He noticed that she has no underwear in now. They ended up doing it in the rv right behind the husbads back.
really want to read this one again. it was a really good story! thanks in advance.