This happened many years ago. My sister and I were always close as we were born a year apart. One night our parents had gone out for the night. She came in my room and laid down on my bed.
She asked me if she could see my 'thingy'. I said what? She wanted to see my 'thingy'. I said sure and took off my shorts. She was laying there and I was kneeling next to her when I said, ok you've seen me now let me see you.
She took off her clothes and laid back down. She then asked if she could touch it. I let her. Felt so much better than my own hand. When she touched me I twitched in her hand and sort of freaked her out. I then took my hand and placed it on hers and showed her how to stroke me.
Didn't take long before I came on her tummy and chest. When I recovered she let me touch her and she had her first orgasm of her life.
After that every weekend our parents went out we would strip nude and masterbate each other.