I thought about sneaking a peek at my sister's friend while she was taking a shower, as she had come to stay over. I quietly went to the changing room... and found that the glass was surprisingly see-through! Her nipples and pubic hair were clearly visible! Plus, she was even masturbating! Before I knew it, I was intensely watching her through the glass, but then her friend opened the door and caught me red-handed! She then rushed over to me and begged me not to tell anyone that she had been masturbating.
