I'm pretty sure my sister doesn't know about this site or she would know I wrote this!
My sister and I have bedrooms that share a bathroom. We each have a door leading into the bathroom. When one of us is in the shower it is common for the other to come into the bathroom and announce their presence to the other, like 'I'm going to brush my teeth ...' The shower curtain is opaque so we don't see each other.
About my sis, she is now 18 and a nice looking young lady too. Last year one day I was showering. In the shower I had decided to get hard and stroke myself for a while, which I often do. For some reason though I prefer to not cum in the shower, but to finish myself off standing over the stool, or into the bathroom sink so I can look at myself cumming in the mirror. I was showering and jacking myself and my sister had come into the bathroom to quickly apply some make-up or lipstick or something, but she didn't tell me she was in there and I had no idea. I turned off the water and pulled the curtain open to dry myself, with the intent to then finish jacking off and cumming into the stool. My sister was standing right there and I was standing there with a giant boner. She looked and gasped and said 'sorry!!'
Then she looked at my dick and said 'wow!' I was embarrassed and she was embarrasssed. She asked me if I had been jacking off in the shower. I said I had been and I told her that due to some of her long showers I think she had been finding some creative ways to use the shower head too! She blushed.
'Go ahead and finish!' she said. For some reason, standing there naked and hard in front of my sister was suddenly erotic! I stood in front of the stool, arched my back and protruded my hips and jacked myself to a fantastic cum, squirting cum on the rim and into the stool.
'That was HOT, bro!' she said. She then left the room and left the house for wherever she was going. The next night our parents had gone out for the evening and she said 'bro, are you going to take another shower?' and she gave me a wink and a smile. I could feel myself getting hard right then. I said 'yeah, you want to get in with me?' It didn't take a minute and we were both naked and in the shower together. My sister has a killer body and it was the first time I've seen her naked since we were about five years old. My dick was aching it was so hard. As she took my dick and started stroking it my hands explored her body thoroughly and my fingers found their way inside her. I fingered the hell out of her and she furiously jacked me off. I squirted cum all over ourselves there in the shower.
Ever since then, I bet she has jacked me off 50 times and we have taken showers together every chance we get. Of course, we don't kiss and all that stuff, we just get each other off and we both find this to be very exciting, and neither one of us feels it is terribly wrong.