This happened to me when I was about 13 years old. I am now 18.
I was in my sisters room one night watching TV. Out of the blue she asked me if I wanted to play a game. 'which game?' I asked. She told me that she would hide an eraser(rubber) anywhere in her body and I would have to look for it. She told me to turn around while she hides the rubber. When she was done she told me to turn around. I started going through her pockets..I couldn't find anything. I told her I gave up then she told me that I should look properly and she told me to feel up her top. I was only 13 so I didn't know what was appropriate or not. She was 17 at the time. Then she told me that I should look under her knickers which I did but I was still confused I hadn't found anything. Then she pulled out the rubber from her sock.
It was now my turn. I hid the eraser in my sock as well. She came over and told me to lie on the bed. I lay down on the bed straight away. She kept saying things like. 'oh where have you hidden it?' I kept laughing. She then went under my pants and started feeling my penis. It tickled so much that she had to tell me to stop laughing. Then after about two minutes of my sister feeling my penis I got a very big scare. My penis started to somehow grow?. I told my sister and my sister just said, 'sshh...its ok it's normal just let me do it' I told her ok because it felt very nice. Then after two to three minutes or so I felt like I was going to piss myself not knowing it was going to be my first orgasm. My sister stroked it fast and stared right at me saying, 'it's ok just let it out it's not pee'. After I came my sister told me that we will play this game a lot...and that we did.