I'm from Europe, and when I was 14 (my brother was 16) I was on vacation with him and my parents in Switzerland. We stayed there for 3 weeks, and I had a vacation boyfriend and he had a vacation girlfriend... on the last evening of vacation, both him and the girl, and me and my "boyfriend" found a little private place behind the shower/machine room... and just a few meters away from each other we kissed our lovers... my brother, always being veeeery protective towards me, warned my boyfriend not to go too far with his little sister (yes, that's me!).

I kissed my boyfriend, and he felt underneath my shirt... (first time a guy touched my breasts!)... and it was semi-dark outside, and when I look sideways towards my brother and his girlfriend, I noticed she had his bermuda's unzipped... and was jerking his erect penis while they were kissing passionately! Kinda shocking to see, and my heart raced like crazy... I never saw my brother's (or any other) penis before, not even in a flacid state, and now, this girl was arousing him, and jerking him.

My brother had his hands on her lower body as well, but I couldn't see exactly (if he was masturbating her as well or not)... anyhow, my brother look at us and warned my boyfriend again (haha, my boyfriend was kinda afraid of my big brother!)... and my brother moved around the corner with his girlfriend, as apparently he didn't feel too comfy with me around. ;-)

(By the way, I did not jerk my boyfriend off back then, I was way too shy... but it was kinda shocking and semi-interesting to see that my brother got a handjob! We even talked about it afterwards... not too much in detail, but I guess his girlfriend made him cum)

And for the record: I am not and never was sexually interested in my brother, but it was just interesting for me to see an erect penis at that time.