My older sister had no idea that I knew her little secret. She might have guessed though that her nerdy kid brother who was so interested in bugging equipment might have been trying some out.
My first clue was when Cath would go to the bathroom the moment she got home from school. Nothing unusual in that, apart from how long she would be in there. She wasn't taking a bath or shower and when she came out she looked bright red, and sometimes had most of her clothes over her arm.
So, I fixed up a little microphone. Sadly video was not developed enough then, but my mic gave me whispered moans and sighs, plus a few whispered swear words and, gold dust, "oh fuck...cumming" I knew my kid sis was rubbing one out.
I admit, sometimes, always on a night when I knew she had Jilled one off, I would take her panties from the laundry and smell her pussy.
In time she got a boyfriend, but our mom would never let them sleep together in our house so Cath used the top bunk in my room. I hated the invasion.
One night in the darkness she said that she knew I used her panties. "I know because mom asked me why there was sperm in my panties." There was an embarrassed silence and I muttered something about being curious.
I heard movement and her arm appeared and dangling from her fingers was the panties she had just slithered out of. "Here. Knock yourself out"
They were still warm, smelled really horny and had a faint after hint of pee. I thought I would go for it and masturbated with my sister in the bunk above me. I was well into it when she said "catch it in the crotch of my panties" I blew my load and then her hand appeared again and I handed them back,
There was a lot of movement above me and the usual whispers of filthy language and finally she said she was gonna cum.
Then she said "Well, now I know what you smell and taste like. I just came pushing your sperm into my cunt. We must do this again."
And over the next few months..........