This happened when we were teenagers
I was reading the bro and sis vacation story that was posted earlier and it remind me of some stuff that happened as a teenager. I too had some interesting experiences with my sister when we were teenagers. I was 16 at the time and my sister was 14 and it was actually her who sort of initiated things. We had always been close and talked about a lot of stuff but I never thought of her in a sexual way.
One day we were watching a movie with some sex scenes in it and she came out and asked me if I had 'lost it' yet. I felt comfortable talking about it and told her the truth, no. Then we got to talking about sex in general and she came out and asked me if I masturbated much. I was pretty surprised (and embarrassed), but told her that yea, I do it sometimes to relieve the tension. I then got curious and asked her if she did it and she didn't want to answer. Instead she started asking me a bunch more questions, if a lot of stuff comes out when a boy does it and what its like when it comes out. Our talking this way was starting to get me turned on and so I asked her again if she masturbated and she finally admitted that she did. I tried to find out how often but at first she didn't want to say anything, but as we talked we both finally just admitted that we masturbate a lot.
She kept asking me questions about what its like when a boy cums, and so I finally just blurted out that I'd show her if she wants to see but that she's gonna have to do it too and let me watch. She thought about it for a while and finally said ok, that she was dying to see how boys did it and what it looks like.
We waited for a day when our parents were gonna be gone for a while and went to her room and started to take our clothes off. We got down to our underwear and I already had a hard on just thinking about what was going to happen. I'd never seen my sis naked before. I took off my underwear really fast and then she took off her bra and paraded around a bit just looking at me and was nervous. Finally she just pulled down her panties and I got a look at her nice light brown bush and her cute ass. I thought I was gonna cum right then. I told her it wasn't going to take long and that she should probably start too because I knew I was gonna cum fast. I sat on the side of her bed and put a bathroom towel down on the floor and started jerking off. I started cumming pretty quick and she was so into watching it come out and was sitting next to me and starting to touch herself. It didn't take her long either and she came and I got to watch her rubbing her clit like crazy.
We did it together a lot over the next few weeks and explored each other a lot naked and how we liked to masturbate. We slowed down after a while and maybe a few months would go by and we wouldn't do anything. Then one of us would get horny and we say 'hey wanna do it Saturday after mom and dad leave', and then we'd have one of our sessions. All we ever did was masturbate, nothing more. It lasted through our teen years and even after I went off to college and came home on breaks. Yea, I think a lot of brothers and sisters fool around at times while their growing up.