After this story my sister became one of my best friends.
My sister and I were two years apart in age, but worlds apart in personality. As little kids we were close, but as we grew and found our own interests we grew apart. I was more introverted, into computers and playing Magic the Gathering with my friends, she was extroverted, socially popular and into sports. Needless to say as we both approached puberty we didn't have a lot to do with each other.
When she came to high school she was instantly popular and was making a lot of older friends and hanging out with boys, I had made out with a couple of girls by then but hadn't really done anything else, I was shy and the kind of friends I had really didn't hang out with girls.
About three months into the first semester I came home later than her after school and as I walked past her door I heard her crying, knocking I walked in to find her on her bed and when I asked her what was wrong she first told me to rack off, not accepting that I decided to ask who hurt her when she said it wasn't anyone that had hurt her she was just embarrassed at school, on further questioning she had been making out with a guy in the year above her when he wanted her to give her a hand job, she got scared because she didn't really know how to do it and was a bit overwhelmed and said no, so he had spread a rumor she was frigid to his mates.
I sat down with her and told her that he was an idiot, if she wasn't ready that was her choice and that it wasn't that hard to learn anyway, I said if he really cared anyway he would have showed her how to do it, as anyone who masturbates should be able to show her. She laughed at that, masturbation was always considered a bit of an embarrassing thing at school and people got teased if they admitted it, she asked me how do I know he masturbates and I told her pretty much everyone does it, they just don't admit it. She then burst out laughing and said "what you too?", which I admitted I did. She just started laughing and we left her room, nothing happened between us that day but our dynamic changed, we became a lot closer around each other around the house even though we didn't really speak at school.
A month or so later we were walking in after school getting the mail there was a catalogue for a local bra shop, she threw at me saying "here's some wanking material" and burst out laughing, I caught it said thanks and put it in my backpack which caused even more laughter. She then joked about when I would use it and I just replied after dinner some time. Although we treated the conversation was a joke, she kept asking questions as we got afternoon tea, how many times a week I do it, where do I do it, do I have magazines, what do I think about, which girls at school I think about, I was equally curious so I asked if she did it, surprisingly she laughed and said yes, although for her not very often, finally she asked if I did it in the computer room which was why I shut the door and I burst out laughing and admitted yeah because the computer had porn. This started a whole new conversation on porn and I told her if she wanted to watch a guy jerk off so she could learn, porn was her best option, she wanted me to show her how to find it so after dinner I agreed to show her how to find porn videos on newsgroups(this was the 90's).
After dinner we settled into the computer room and told our parents I was showing her how to format her English homework, they would always leave us alone if we were doing homework, browsing through news groups looking for videos of guys jerking off was not my usual type of porn but it was funny watching the expressions of my sister as each badly pirated one to two minute movie clip played. After we had seen a number of videos she joked which dick looked like mine. I wasn't much of a match for porno dick so we navigated to a picture site and started browsing cock picks, laughing I finally found one that looked like mine, we were both in hysterics but I was also pretty turned on. I then pulled up a site with women and told her to find her boobs, intently scrolling she finally found a pic of a woman with small firm breasts with largish nipples, I thought it was hot, so I started inquiring more about her boobs, what bra size how they felt when she ran, she answered every question with enthusiasm. Finally I dared her that we reveal if we were right about matching the photos, she gasped and said "what show each other" and I laughed saying yep, its the only way to be sure we aren't lying. She agreed and we both agreed that we both had to turn around and close our eyes, I had to pull my pants down and she had to pull her t-shirt over her head, then count to three we had to turn around and open our eyes, on the three count we both turned around, I was pretty hard and she burst out laughing, seeing a real pair of boobs for the first time was hot but also we were both just having fun so I was laughing as well, it just felt natural. I went to pull my pants up and she said wait, and made me stand on the chair level with the computer monitor so she could closely compare my dick to the picture, giggling, she said yep pretty close, although joked that I might have been a little smaller, slightly offended I made her stand next to the computer and compared her breasts, I looked at the PC and then closely inspected hers and as I said "yep pretty close but yours are smaller too" she gasped and grabbed them saying they were not, so I grabbed them and I guess you are right, she laughed and then gave my dick a squeeze and said "just evening things up".
Finally I looked at my cock and said I should probably go to my room and look at the bra magazine when she asked if she could watch and said that she'd seen it now she may as well show me the end, at this point I didn't care that my sister was watching, so I pulled my shirt over my head and I told her if I was going to be naked she had to be as well, laughing she slid out of her tracksuit pants and sat on the chair next to me and said "well go on". I started stroking my dick in front of her, I wasn't going to last long as I was so worked up but she was enjoying every minute of it, she even started playing with herself a little bit, but she was more focused on what I was doing rather then herself, finally I was about to cum so I told her, she got closer to my dick and I came all over my stomach and chest, she giggled as I came and then sat there watching as my dick slowly turned limp. I should have felt embarrassed at this point but I was just happy I could have shared that, she put her clothes back on and went and got me a rag and watched me clean up and put my clothes back on and said that was fun, I agreed and told her that at least she should now know how to jerk off a guy.
This was the only time this happened between us, but this changed us and brought us closer together, growing up through high school we had no secrets and would tell each other everything, we weren't modest around each other and nudity wasn't a problem, we would tell each other when we would masturbate and shared our experiences we had with other people but there was never a need to do anything sexual with each other, the first time was a lesson for the both of us and that was all we needed.