Watching my s****r SeXStoRY

I used to get myself off watching my older s****r having sex. I once heard Angie and her boyfriend fucking when they thought they had the house to themselves and got really turned on by it, and eventually thought of a way to get to see them in action. So I learned to tell them I wouldn't be home until late and then to hide under her bed and wait for the two of them to come home and get busy upstairs - usually straight away. She was really in love with this guy, and I used to hear her masturbating in her room whenever she couldn't have him inside her. I was so into the thought of seeing them together, on the first occasion my cock got erect as soon as I heard the taxi pull up outside and the front door opening. I looked through the blinds of her bedroom to see what she was wearing and watched as her boyfriend touched her up as they came up the front steps - readying her for the bedroom. So I knew I was going to be in luck. Even before I heard them coming up the stairs, my cock was fully erect and I was masturbating hard. But I guess I learned more from my s****r than I thought I would. I thought it was straightforward, honest to goodness sex they were having together. My s****r was always very prim and proper with me, so it came as a surprise on the second session to see her getting dressed up as a nurse in short white skirt and black stockings, pulling down her boyfriend's zip to "examine" him. I can still recall her saying "I'll see what's wrong and see if I can get you straightened out", giggling as she masturbated him. And then measuring him with a steel ruler to see if his cock was "healthy". On other occasions she was a bunny girl, a French maid, a secretary in a pencil skirt, and a 1950s rocker in a petticoat. But always ending up with him licking her and fucking her still dressed in those outfits. And that's how I got hooked on costume play, I think. Without it now sex is dull and boring. A new girlfriend, like the one I'm with at the moment, always offers a challenge....