When I was in my early 20s I had moved to the 'City' and my sister, who was 10 years older, happened to live 10-15 minutes away. Although she had been married for 10 years and had 2 children, she worked out and still had a fairly nice figure.
One Saturday morning she stopped by to pick me up and we ran a few errands. It was one of those spring days that you are torn between wearing a sweatshirt or t-shirt. I happened to pick the sweatshirt and after about an hour decided I need something lighter. Since we weren't too far from my apartment, we drove back so I could change.
I opened the apartment door and while walking to the bedroom started taking off my sweatshirt so by the time I reached the closet, I was bare-chested. When I found a shirt, I grabbed it off the hanger and turned around. Surprisingly, my sister was standing there. I figured she would just wait in the living room. This instantly brought back a memory of when I was 5 or 6 and in my sister room's one day when she decided to change her pants. She had pulled down her pants and while doing so, also pulled down her panties. I remember looking at the dark patch between her legs, which seemed like forever, before she pulled up her panties.
Suddenly, I told her I was going to put on a pair of shorts. I unbuttoned my jeans and slid them off while at the same time slid off my underwear so I was standing in front of her with my jeans and underwear at my ankles. I was happy to find that she didn't leave or turn her back. I was quickly gaining an erection, to which she commented that I must be happy. I told her that I would be much happier if I came and slowly started stroking myself and massaging my balls.
She took off her top and bra, knelt in front of me and watch me stroke myself. This was the first time I had ever had anyone watch me masturbate and was getting real excited. While I stroked myself, she would lick the precum from my penis and leave a little saliva on the head which I would use as lubricant. She pulled her panties down and started rubbing her clit with one hand and massaging her nipple with the other while still occasionally licking the precum from my head. This went on for about 10 minutes before I came all over her breasts. It was so thrilling to come all over her and watch my semen run down her stomach and into her pubic hair. I then bent down and licked and sucked on her nipples, picking up a few spurts of my own semen in the process. I could tell she really enjoyed me licking and sucking her nipples and shortly after she had an orgasm. We quickly cleaned up and went on to finish our errands.
We never talked about this, though a few times I showed up at her house when her kids were at school and husband was at work, but we never had an more adventures.