My sister is two years older than me and has a great body with a really nice butt. She always wears tight jeans and little thong panties or no panties at all. I've stared at her butt in those jeans many times.
One day when our parents were out, I was upstairs in my room and decided to go get something to drink from the kitchen. From the top of the stairs I could see my sister doing something in the living room. She was laying on her stomach on the floor in front of the TV. She was wearing an older pair of skin tight jeans and she was humping and grinding herself against one of the sofa cushions that she had squeezed between her legs.
I watched her as she continued to gyrate her hips against the cushion. My cock was hard as a rock as I rubbed it through the front of my shorts. Within 30 seconds I had shot a huge load into my shorts. My sister must have cum, because she was now laying still.
At this point I just went back into my room and and jacked off 3 more times thinking about what I had just seen.
Later on I heard my sister get ready for bed and go to her room. I got up to go to the bathroom, while I was in there I decided to take a look inside the laundry hamper. I almost passed out from excitement when I saw her old dirty jeans right on top. I put the crotch to my nose and got an instant hard on. The crotch of the jeans was musky and damp from her juices. I laid out the jeans on the bathroom floor with the butt side facing me, then I layed down on my stomach and fucked my sisters jeans. I came twice on the jeans before returning them to the hamper.