It might help to read part one of this story to know how things progressed to where they are.
Sarah is the 20 year old sister of Katie, who is 23 and Brian, her brother, is almost 19. Sarah had came home and walked in on Brian and Katie.
Sarah stood and quietly watched Brian fucking Katie. They were so caught up in the heat of the moment that Sarah's presence went unnoticed until after they reached a mind blowing orgasm together.
Katie saw her from the corner of her eye and shrieked.
"Is this a private party?" asked Sarah.
"Uhhhhhh, it was supposed to be," stuttered Katie.
"God Katie, you are such a whore but I never thought you'd do the nasty with our own brother," Sarah glared at her sister.
Brian and Katie sat upright on the couch. "There's a perfectly good reason, my dear sister."
Sarah couldn't believe what she was seeing. They just sat there, shamelessly nude side by side. "What reason might that be?"
"You know Brian is leaving for Iraq in 2 weeks. I couldn't stand to see him go still being a virgin."
Sarah looked surprised. "Brian? You dated Jennie for 4 years and she never put out?"
Brian looked sheepishly at his sister, "Nope, never once. Nothin'."
"Well, that's still no reason for this. How long has this been going on?" Sarah asked.
"This was the first time but it sure won't be the last," Katie replied as she caressed her brother's cock. "As a matter of fact as soon as he can get it up I'm going to fuck him again and every chance I get for the next two weeks. You can stay and watch, join in or what ever you want"
"You better not let mom and dad catch you. They'll be home soon. I'm going to take a shower and go out with some friends."
Sarah went to her room and prepared for her evening out. She couldn't get her siblings out of her mind. Brian did have a nice dick. Ewwwwwww, she thought, how could she even think such a thing.