When my sister and I were younger, we would fool around once in a while if our parents were not around. When I was about thirteen and she was twelve, we would strip down to our underwear and play with each other in my bed. Once, we discussed having sex but decided that if we were not careful, my sister may end up pregnant so my sister wouldn't remove her underwear while we were fooling around.
I would start by kissing her and running my fingers around the outside of her bra and panties. Then when her nipples got hard, I would pull her bra off and suck on them. She would wrap her hand around my cock and hang on to it as we rubbed against each other.
After a while, I would feel like I had to cum. I was a masturbator so I knew the sensation, and she would remove her panties and she would start fingering herself while I pulled myself off.
Even though we were very close, this ended when we got into high school, but I still think about it every once in a while.