My 19-year-old sis has been keeping a secret from the rest of the family, she's bi. I was home early from school last week because I was concussed in games and walked up to my room at the back of the house overlooking the garden.
Our garden is surrounded by trees and we live in a remote area. It was really sunny. I opened my window to let in some fresh air, and heard some muffled noises like an animal struggling. I looked and saw two naked girls in the garden! One was my sister. She was supposed to be in College but she was on her back with another girl between her legs fingering her.
I pulled my venetian blind almost shut so they wouldn't see me, and got my cock out and started to pump. Soon I shot my load up the radiator. They were whimpering, I think they both came. It was so hot seeing my own sister come! I am going to 'injure' myself next week in case they play again.