My brother is aged 13 and I am 15 year old girl.
We both sleep in the same room but on separate beds at both ends of the room.
I am a sound sleeper and wakes only in the morning too late.
One night I had bad dream and just opened my eyes stark awake and as I was lying on my side facing the brother's bed opposite I astonishing saw the horrible act in the moonlight coming through the window.
My brother was lying on his back with his shorts down his ankle and his hand was moving up and down his crotch.
To my surprise I saw his erect penis when he changed hands and was stroking it.
He was making such movement and with some hissing sound I saw him jerking too fast and something ejecting from his penis was his semen shooting up and falling on his stomach.
His eyes were closed and he was enjoying.
While he lay relaxed his erect penis shrunk and slowly became small.
I was shocked to see such an act.