Well let's say we were younger, about the time I hit puberty my mom and dad quit walking around nude. But that did not stop my sister and I we are not that far in age so after a year of growing pubic hair I saw my sister was getting a little dark crop down there. The summer I turned 13 my parents let me stay home and watch my sister while they were at work.
My mom would knock on my door in the morning to tell me good bye or I would hear them getting ready. I slept in the nude and she would walk in or my dad would, like I said they did not walk around the house as much but we were still an open family. After they left for work my sister would climb into bed with me and fall back to sleep. One morning I woke up with a raging hard on and my sis was still out so I got up and went into the bathroom to give myself a tug. She must have woken as I got up and trailed me, she walks in and asks what I was doing, I told her that pulling on it felt good. She asked to watch and her standing there nude with little tits and starting to grow pubic hair I said sure.
As I was masterbating she told me that she touched herself from time to time and it felt weird but good. I asked her to show me and she started to play with herself. I asked her where she learned to do that and she told me she saw mom doing it one night when dad was out of town.
The rest of the summer and till now we masterbated together. I have more from when we got a littler older and now that we are in our twenties.