I Watched My Mentally Challenged Sister

My sister was a forceps baby smart but slow. We hung out as kids and would ask strangers for money to buy sweets candy etc. one winter night we ran into a guy who we knew from church. He was married with a few kids and we asked him for a quarter. He beckoned her to a dark schoolyard with an agreement of giving her $.50. Back then that was a lot of money for a kid as things were rough. He laid her by the hand not paying any attention to me and she had a sway to her Like she knew what was going on. Once inside in the door corner he started kissing her and to my surprise she was kissing him back. He started feeling her up and dish with exciting me as I never saw her like this and feeling her up and really getting into her and she was given his good if she was getting finally laying her down he fucked her and she was fucking him back. I jerked off many nights thinking of what happened that night and I’m pretty sure they had done that before. She was 16 at the time I was 14.