Grandma, Mom, and another Surprise?

I had just finished dinner and Dad had just gone of to work, he works night shift. I was up stairs in my bedroom, I was horny as hell, after staring at moms cleavage, but I knew what was behind the blouse now, So I was actually looking at her bare breasts, just everyone else saw her blouse. I just got her panties out of my hiding spot, and was about to start wanking, when I heard what sounded like furniture being moved. I thought WTF? Dad was at work, so it meant Mom was struggling to move something by her self, so I put back the panties, carefully zipped up and went back down stairs to investigate and offer assistance. As I was approaching The master bedroom, I could hear what sounded like muffled screaming, so I carefully opened the door a crack, hoping not to find a burglar, but I was ready to burst in, if it was. What I saw shocked me to the core, but at the same time I became rapidly aroused. I saw Grandma, Mom and a young girl, which I could not identify. Grandma was behind this girl, with a strap on dildo, which had to have been 10 inches long and a good 2 inches diameter. Mom was kneeling at the girls head, who was busy, eating Moms pussy. Granny was pounding the hell out of this girl, who was of course on all fours, and her screams were muffled, by virtue of the fact that she had her mouth, firmly entrenched in Moms pussy. I was now working my cock furiously, at the scene played out before me. I watched for a while, but it was too much for me, I quietly walked in the room, Mom saw me right away, but didn’t even flinch, as I walked up behind Grandma, Dropped my pants and started to insert my cock into grandmas ass, and she didn’t flinch, but after a few strokes, she turned her head to face me, and whispered, I should take over from her and she would get behind Mom. I did exactly that and the girl did not notice or see the exchange take place. I kept on banging the girl, with the same speed and force as Grandma. Grandma gently pushed Mom in the back, indicating for Mom to go on all fours over the girls shoulders and back. Then Grandma was able to insert her strap on into Moms ass.