This happened a little over 10 years ago. My sister is about seven years older than me, and it was the year before she went off to college.
I believe it was shortly after I had discovered masturbation. I was pulling on my dick a few times a day, and I wasn't producing semen yet, so there was never any mess. I didn't even have pubes yet, really. Pretty much any chance I got, though, I was jerking my little dick. I had learned how to do it from my friend. We never did it together, he just told me what happened and I took it from there. He was also the one who showed me my first porn. It was a picture he'd printed off his dad's computer, of a topless girl with wet panties. His parents found the picture and he got in trouble so I wasn't allowed over there for a long time when he was grounded. I think they were pretty strict about the whole thing because his mom was a religious nutcase.
So during that time I was still as horny as ever, and now had no access to even a single picture of porn. We didn't have a computer or anything, and I couldn't go buy a magazine. My imagination only took me so far and I had urges I didn't know how to satisfy.
I was a younger brother who enjoyed generally being a nuisance to my older sister, who wanted almost nothing to do with me. The age difference between us kept us from really being good friends, so I was often left out when she had her friends over or did anything. They would always be in her room laughing and being loud during sleepovers and I was jealous that I couldn't have fun with them. So, one day, I snuck into her room. I began snooping around, just being curious, wanting to know what it was in here that was so much fun. Then I saw them: in her laundry hamper was a pair of panties that looked just like the pair the girl in the picture I saw had on. They were almost the exact same, just pink cotton panties, but they had a little bow in the middle on the waistband (in hindsight, they probably weren't that close to what the girl in the picture was wearing, but all I had experience of was pink panties).
I picked them up from the hamper and my first thought was to put them on. I quietly returned to my room. Being one of two children in a single-parent household, I was often given a decent amount of privacy. My mom didn't bug me as long as I wasn't getting into trouble, and my sister didn't want that much to do with me. I closed my bedroom door and stripped naked. I slid the panties up my legs and over my little erect dick. I wasn't very big at the time. I would masturbate with two fingers and a thumb. My dick pointed up inside the panties, barely stretching the fabric. They fit me pretty well, as size-wise I was probably about the same as my sister at the time.
I looked at myself in the mirror in my room and felt so horny. My heart was racing so fast. I pulled my dick out of the leg of the panties and began to jerk. In almost no-time I was cumming, and I produced my first semen. It was barely bigger than a drop but I was amazed and played with it between my fingers before rubbing it off on my leg. I didn't want to give the panties back, so I put them under my mattress. All day long I would take them out, put them on, pull my dick out through the side, and jerk. I probably had four orgasms. I decided to wear them to bed, which, might've been a mistake. My sister returned home late that night.
Apparently she was well aware of what she had done with her panties, and noticed that they were missing from the very top of her hamper. Her being the only one awake, she came into my room and woke me up, asking if I'd been in her room. I was very sleepy, but when she asked me that my heart started racing. It probably wasn't too hard for her to tell I was lying. She asked what I was doing in there, and I didn't want to tell her. She told me she knew her panties were missing and I finally caved and told her I took them because they looked like the ones in the picture. I was begging her not to tell our mom and trying to bargain with chores and all that. She knew she had me by the balls. She asked if she could have them back and I told her no. I was too embarrassed to tell her that I was currently wearing them. She tried to pull me out of bed to get them and saw that I had them on. The begging, pleading, and bargaining started again.
She just looked at me, holding my sheets up, despite my efforts to cover myself. I can't remember exactly what was said but she smiled at me and basically told me I had to do whatever she said or she'd tell our mom. She brought me back to her room, and wouldn't let me take the panties off. She sat me down on the end of her bed and started pulling out clothes. She said something like, if I wanted to wear girl's clothes, I had to do it properly, and she wouldn't take any arguments, which were quickly silenced with a, 'Do you want me to tell mom?'
She dressed me up in thigh-high socks and a pink bra. The bra was tight and the cups jutted out a bit. She stuffed it with socks. She then took out a white dress with these blue flower patterns and I reluctantly allowed her to dress me in it. She put black heels on my feet, which were a little snug. I remember I could barely stand in them as she asked me to model for her. She said something was missing and then got up really fast, coming back with her make-up. Once again I had to let her put make-up on me, and she would go, 'Oh, don't you just look so pretty!' and other stuff like that. I could see in her mirror I looked exactly like a girl now, except my short hair.
She was on her bed and patted it, beckoning me over. I climbed on and she had me lay down on my back. She took something out from under her bed and asked me if I knew what it was. Of course today I know it was a plastic purple vibrator, but then it just looked like a pen to me. She told me to close my eyes and keep my hands at my sides. The buzzing started and she told me to keep my eyes shut, or she'd tell. She began running the vibe up my already-hard little dick through the panties. It felt incredible when she would rub it on the head. She was lifting my skirt and playing with me for only a couple minutes or so, and I had my knees bent, with my legs spread. I was grabbing her sheets with my hands, barely able to take the sensations she was providing. She was whispering, 'Come for me, Nicky!' as my hips bucked and I made a small wet spot in her panties where my little dribble of cum came out.
I was out of breath, laying on my back, having experienced the best orgasm of my life (at the time), all dressed up as a girl. She told me to stay still and not move a muscle. I watched as she quickly slid her pants and panties down and began to toy herself with the same vibe. She was constantly running her hand over my legs, and would reach under my skirt and touch my penis in the panties. She'd feel my 'breasts' and squeeze them. She lifted up the skirt and just looked at me while she was toying her pussy. I had never seen a pussy before, so seeing my sister's pussy got me horny all over again. My dick was poking into the fabric of the panties again.
My sister took notice and grasped my dick between her fingers through the panties, and began to stroke. She put the vibrator on my leg and was humping it while she jerked me off. The friction with the panties was almost more than I could take, and it wasn't long before we both came at nearly the same time. Afterwards, she cuddled me for a bit and told me how I could never tell anyone what we're doing. She still used the same old threat of telling our mom. Taking the dress off, and the socks out of the bra I was wearing, she told me to go back to my room. She sent me to sleep in the bra, panties, and thigh-highs.
There's a bit more that happens after this that I'll probably put in another story soon.