My mother and I
 

My mother and I never actually had sex, but she used to masturbate me...

That is pretty much what I do with my 19 yo step-daughter. I know sooner or later something will happen. :)

We never did anything, but I saw my dad masturbating at the computer once. He had headphones on, and didn't know I had come home early from shopping.

When I was about 14 or 15 my parents dragged me to all family events, but I was not quite old enough to be with the grownups, and too young to play with my mostly young cousins.
O had one cousin, Angela, who was 2 years younger than me, and more than a little chubby. that meant that even at 14 she had a rack.. one evening we were left in a room while the adults talked and drank, and she let me feel her up, under her shirt, then in her pants.. I ended up fingering her, no idea if she came or not (what did I know :) ) but I sure as shit enjoyed it a lot

Do you remember what he was looking at? I'd be very interested to find out.. and are you sure he did not know you were there? :)

And how did you feel/what did you do after seeing that? ;-)

My sisters and I would sometimes share bathroom time together, typical in our small house at the time. Very common for 2 of us to be in the shower at the same time, almost all the time there was at least one of us in the shower and the other in front of the mirror doing something. My oldest sister had no shame about when she masterbated. Embarrassing the first few times, then normal.