Editor's note: this story contains scenes of non-consensual or reluctant sex.
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My 20 y/o sister left her computer on recently and I discovered she had written some stories for literotica under the name Surely, or Surly Shirley. Her stories were not particularly exciting for me and did not indicate any interest in sex with me, but the stories she had flagged seemed to mostly about people who were sort of showing off one way or another. Exhibitionism! I wondered if she had ever tried it. I decided to play a trick on her. The next time she was taking a shower, I was waiting when she stepped out and I slid a bag over her head- it was a big bag and I pulled it down to below her butt and tied it between her legs, so she was basically tied in a bag. I had put some towels in it so she could dry off and it wasn't cold. She is not very heavy and I carried her downstairs to the living room, knowing that my parents were at a party and would not be home until late.
I had built a sort of "stocks" device and could put her legs in it while the bag was still on her. The openings were designed to fit around her lower thigh, just above the knee, so I could loosely tie her ankles to the other side and she was not going to get out, even if her hands were free. Her butt was supported by a narrow ledge on the "body side" of the stock, but was narrow enough that with her legs far apart (as they were- the leg openings were almost 12" apart!) so her butt was almost totally exposed, as was her perineum. Her arms had separate holes and she didn't really try very hard to hurt me as I got them thru the holes and tied together with soft rope.
I had set up the device so I could open the picture window in the living room and anyone walking by outside would see her naked back and butt as they walked by. And if I rotated the whole thing, they could see the pink of her inner labia because her legs were apart far enough (and she shaved her pubic hair). I had a bright light to shine on her- it was activated by motion of anyone outside, which would also draw their attention. Did I mention that she was making a lot of noise, begging, threatening, and asking nicely to be released. She did not really fight though.
What she didn't realize was that the entire device was hinged so that it could tip forward, which put her on her knees, and the seat ledge could be removed so that her rear end was totally exposed to the window. I saved that for the second person who walked by... the second MALE who walked by... I didn't need some nosy woman spoiling my fun.
Although I had not planned on molesting sis, I guess I couldn't resist looking closely at her naked anatomy. She kept her chest up against the device so I could not see her breasts well, but her beautiful rear end and smooth cunt were displayed very well.