I never jacked off with my brother for the simple reason that I never had a brother, but my older sister watched once. I know I've told this story before but it was some years back.
We were returning from a summer in the Colorado moutains in the company of another family, a doctor, his wife and daughter and young son. As it happened we traveled in three cars, one being my dad's Dodge coupe (red wire wheels, rumble seat, the works). My older sister (16) and the doc,s daughter (15) were going to be in the Dodge (sister driving) and I was flattered when they invited me to ride with them. I missed out on getting a window seat and wound up as "Malcomb In The Middle".
I don't recall how long we had been traveling (several hours) and conversation had lagged until Willie (doc's daughter) started telling about all the information about "sex" that she had gleaned from some of her dad's books. When she got to the area of male genitalia and the marvel of progressing from soft and small to hard and big, she cut us in on the hot scoop that, "Some men are as much as SIX INCHES long when they're hard." At this my sister asked (I guessed quite innocently) if I thought I would ever have one that big. With some bravado, I'm sure, I let them know that I already possessed one that was longer than that. Willie gave a snort while she advised me that what I had just claimed was IMpossible because the books she had read suggested that the largest ones were just six inches - most were shorter. We argued a bit till I offered to settle by showing her that I wasn't lying. I looked at my sister for support but she just told me to "Go ahead and show her."
By this time, what with all the sex talk, I already was hard so I slid my pants and underwear down and exposed my pride (which was right at 7"). Willie and my sister both gave it a good scanning and then Willie pointed to the slit and gave us the news that, "Right there is where the baby-making juice comes out." Again my sister asked, "You've never had any of that, have you?" And again we were treated to a hoot from Willie, "You couldn't have because boys can't do that until they're 15 to 17 years old." I had turned 11 about 4 months earlier. I was getting tired of her doubts and I told her that I made it shoot out all the time and that I could do it ANYtime. THAT surprised her. She obviously thought there was only one way to make the juice fly. Now there was a gap in her knowledge so she wanted to know, "How can you MAKE it come out?" Sitting there between two girls with an exposed erection I was very much ready to do something about it so I started to show her. I started with a little pumping action and then went to long strokes. Willie was watching closely and asked, "Is that all you have to do?" I gave her an affirmative and then she wanted to know if it would work if SHE did it. I felt sure that it would so I took my hand away and she replaced it and was stroking the way I had done. My dick was not unused to strange hands but Willie was so obviously impressed and eager that it was only a matter of a minute or less before I breathed, "Here we go!" and I let go two shots that both hit the dashboard. The rest ran down over her hand and she changed hands so that she could do a sniff test on her hand AND a little taste with the tip of her tongue. My cum was about to drip off the dashboard so Willie grabbed a hankie and started to wipe it up when she suddenly dropped the rag and grabbed herself in the crotch. She stiffened out in the seat and after stomping her foot on the floor two or three times began to relax. My sister laughed at her and asked, "Did you have an orgasm?" Willie was breathing a little fast but she managed to nod and admit that she had never had one before and added, "My God, that's the best thing I've ever felt!" She took hold of my dick again (which was on its way to a semi but now became fully hard). "Doesn't that thing ever go down?"
It was at this moment that Sis (who was watching stuff other than her driving) ran off the pavement and when she pulled back a tire blew out. That ended the sex talk as we had to change off the flat and when we resumed our trip we found both our families waiting for us about a mile up the road. Good thing they didn't turn around and come back!
Gramps