What the hell is my sister doing?

It was a Monday morning many years ago. I was getting ready to go to school more excited that normally as it was the final two weeks of my freshmen year in high school. While my little sister was still sleeping because had recently quit public school and begin to do school online. She got in trouble allot at her middle school and was always getting suspended, but now she is home alone doing school on the computer. As usual my mom would take me to school before she would go to work, so we left and at school I had a okay day failing miserably on some exams while passing others with no stress.
Going home from school I had to ride the bus home as my mom was at work and couldn’t pick me up. My bus got me home about 20 minutes earlier than usual and I knew my sister wouldn’t expect me yet so I decided to go to her window and knock on it to surprise her. I walked up to her window expecting to see her on her computer doing school work, not what I was about to see.
I saw my 13 year old sister was lying on her bed with tight ass up in the air and my neighbor was naked with his dick out erect about 5 and half inches. He then pulled down her pants and panties then slowly inserted his dick into her pussy. They were facing the opposite direction me so I was lucky they didn’t see me. I didn’t want them to know I saw, but I wanted to watch so I hide in some next to an abandon house about 7 feet from my sister’s window. I wanted to jerk off to this so bad but knew it was a good idea in public. I realized that I was lucky that I had took my video camera to school that day and I pulled it out and started recording and zoomed in for a better view. He was fucking fast but his dick didn’t go in so far because of her tight cunt. He was grabbing and smacking my sister’s nice ass for about 3 minutes and then they switched positions up and she rode his cock like it was a horse! She was barely holding on to her small teen tits as she moaned loudly. It was none stop fucking for 5 more minutes and then my neighbor pulled out and my sister slobbered all over his dick until he came all over her face.