My family is Catholic and as such we believe in waiting till marriage to have sex. I was the youngest of three (16), my parents (dad 49) and mom (43), two older sisters (18) and (20). My oldest sister was in college so I didn’t see her that often but my other sister and I were still in highschool. I had a late afternoon class which kept me from home but my sister would usually get home around 2pm and she and my mom would do something together while they waited for dad and I to return. One day I didn’t have my normal class so I went home around 2:30, two hours earlier than normal. I wanted to surprise them so I went into the house quiet, put my stuff down and went upstairs. As I got to the top I heard moaning coming from my parents room. The door was slightly open so I went to try and take a peak. At first I thought my dad had come home early and he and mom were upstairs having sex. My sister’s room was empty so she was probably out. My parents were always open about them having sex so hearing or even catching them in the act wasn’t strange to me. What was strange was when I peaked in I saw my mom and sister naked on the bed and our black neighbor Mr. Greene standing there with his head between my sister’s legs as my mom watched over rubbing her vagina and breasts. I couldn’t believe it, my mother was always so against sex before marriage and especially cheating, yet here she was with my sister, her daughter, having sex with our neighbor. At first I got angry thinking about all the times I almost had sex with my girlfriend but she would always stop us, now she’s openly encouraging my sister to fuck openly. The neighbor was now holding my sister as his penis went deep inside my sister’s vagina, her arms and legs wrapped around him. My sister had surprisingly large boobs for a girl her age and they were bouncing as she rode the neighbor. My mother just laid back and spread her legs before rubbing herself. She was a beautiful woman, long blonde hair and big boobs. I had never seen her pussy so open before but it and her ass were stunning to look at. My sister was screaming in pleasure saying how good his cock felt inside her pussy, she even asked my mom if she could let him cum in her. She begged her over and over but my mom said no. As he was about to climax, the man threw my sister off onto the bed and dragged my moms head over to replace her. I watched as my mother sucked on the man’s cock and swallowed his big load of cum. After they finished my mom told my sister that she can’t tell anyone about this, not even her brother. My sister giggled and said she knows and that she hopes she can keep coming back for this. I hurried down stairs and pretended like I just got home. My mom said to go to my room, she’ll be out in a minute. I went to my room, locked the door and started jerking myself off to the image of what I saw. I don’t know what to think, my mom is cheating on my dad and letting my sister join in. Who knows how long this has been going on for. I think I’ll come home early again tomorrow and confront them, maybe they’ll let me join to keep quiet. At least now I have the info I need to fuck my girlfriend without my mother being able to stop us.