When I was 16, my sister (13) had school friend to stay for half term. One day I was passing sis' room and heard giggling. I peeked through a crack in the door. They were on the bed, naked from the waist down. Sis was using a wooden recorder (wind instrument) to masturbate her friend. She was pushing it into that cute little pussy, and I rapidly became aroused. I pulled my cock out and quietly opened the door, tiptoed in with my cock sticking out of my pants. At first they didn't notice. Sis had her back to me and her friends eyes were closed.
Why don't you try the real thing?? I said.
They froze.
I moved closer and gently took the recorder from my sis. They were both staring at my hard cock. I undid my pants and let them drop. The girl had one leg dangling off the bed, and now I knelt there, between her thighs. I took sis' hand and put it on my cock.
Go on, carry on as you were! I said. Moving closer so my cock was hovering over the other girls pussy.
Sis tentatively grasped my cock, and started rubbing it against the girls pussy lips. That felt sooo nice, her silky smooth skin against my shiny tip. Then she pushed it between those lips, and I felt it pop into the girls tight little vagina. I pushed in a little more, and she gave a little moan, then her hand came down and started to masturbate her clit. That was so sexy to watch, and I started to slowly thrust in and out of her, deeper and deeper. Sis was lying beside her now, tongue kissing with fervour. I looked and saw her little pussy with its sparse covering of blond hair. I put my hand on the inside of her thigh, then slid it up and stuck my finger into her, my thumb finding her clit. She looked shocked for a moment, then carried on snogging her friend.
The girl was now writhing in time with my thrusts, and her little fingers were a blur on her clit. Then she went into a moaning spasm, her vagina gripped me, then everything became very wet and slippery. It felt incredible... Half a dozen more thrusts into that slippery heaven, and I felt my sperm welling up... I pulled out hastily and wanked a couple of strokes before it flew out of me all over her sweet little tummy...

That night I returned, and this time my sis got to feel my cock inside her too. She lives abroad now, so I only see her about once a year. She has her own family, but she still has the figure of a teenager, and we always get to fuck when we meet up!