It was a normal saturday afternoon, and my sister (13) had her best friend Jennifer over to spend the night. (She was also 13) My sister wasn't bad looking, but Jen was hot. Perfect boobs, perfect butt, etc.
Anyway it was just the three of us, my parents had gone out, I was checking my email when I heard my sister ask Jen if she wanted to shower first. Jen said they should get one together like they did at dance camp. My sister said 'Sure', and I heard them walking to the bathroom. Then I remembered that due to a mistake while our house was being made, anyone who crawled into the attic could see perfectly into the shower through an AC vent, and had a good view of the rest of the bathroom.
So I quietly went right up there and saw the two of them in front of the mirror already in bras and thongs, and I heard Jen saying, 'But your's are bigger than mine', and my sis saying, 'Not by much, and yours fit your body better', Jen said 'Whatever'. Then they took their bras off and I saw Jen's boobs for the first time, it was amazing. Then Jen said, 'So what are your favorite kind of panties'? 'Thongs and g-strings', my sister said, 'Yours'? 'I like thongs the best also', Jen said, 'I thought they would be uncomfortable at first, you know with it going up your butt and all, but now I love them'.
Then they both removed their thongs, and I was in heaven, as my sister turned on the water, Jen said something I didn't hear and my sister slapped her butt playfully causing Jen to put both of her hands on her butt which was realy erotic. I watched their whole shower, which included them both masturbating to orgasm after they both agreed they did in fact masturbate. I will never forget this day and hope to see this happen again soon.