When I was 13 years old, just a few months after I had ejaculated for the first time, and with maybe six thin curly crotch hairs to my name, I was in front of our house riding bikes with four friends, three guys and a girl, all between 13 and 15 years old. We had set up a little jump, a piece of plywood over a brick, and were running our bikes over it. My turn came again, and as I was riding up, the plywood slipped, so instead of the bike going in the air properly, it kind of tipped forward, causing the seat to hit me in the balls. I yelped in pain. My sister, two years older than me at 15, came running out of the house when she heard me. She was always the protective one.
She saw me curled up in a little ball holding my crotch. She asked my friends, now grouped around me in a semi-circle, what had happened.
Without asking or saying anything, she unzipped and pulled down my shorts and underwear, in front of all my friends. I was horrified, but I was also in pain and a bit relieved, figuring that help, if needed, was at hand.
Fortunately, the pain was starting to subside. My sister laid me out on my back, since I could now uncurl from my fetal position, and she very gently took each ball, one at a time, in her fingertips and felt them, probably looking for swelling or, horrors, a fracture. I must not have been in too bad shape, because although her touch was painful, it wasn't impossible to bear. In fact, it felt, well, I don't know, interesting. To this day, I remember that her fingertips on my scrotum were deliciously cold.
Then she casually announced, 'Well, there's only one way to discover if everything still works OK.' And right there, in front of my friends, she started jerking me off! My crazy sister just started gently pulling on my dick, and of course, it started to get hard. In fact, within less than a minute, it became rock hard, and I squirted my cum all over her hand. She simply said, 'you're OK,' and walked away.
It took a few days before I was really able to talk about it with my friends who had been there. And, then, to all my other friends who heard about it. At first I was quite embarrassed, as if I had done something wrong. But the truth is, almost every single friend said they wished it had happened to them, pain and all. Even the girls said they'd like to have had a, well, similar experience, or to have been my sister in that situation.
Although my sister, now married with two kids older than we were then, has always been a bit of a tomboy, and willing to try anything (arm wrestling, motorcycle riding, stamp collecting, guitar, you name it), she never did anything remotely like that ever again. When I tried to talk with her about it, even 30 years later, she never has said even one sentence about that time.