Huge Sibling Secret Discovered

This happened just yesterday and last night. I am choosing to remain anonymous as well as everyone involved out of respect. I have 4 siblings; 2 younger sisters ages 14 and 17, and 2 older brothers ages 20 and 23. I am right in the middle of them all at 19, started college in August and live at home. My oldest brother lives on his own but the younger of the brothers lives at home and is sort of lazy and doesn’t do much other than work a part time job 20 hours a week. Both sisters also live at home obviously. My parents work very different schedules and one of them is almost always home. My aunt went to the hospital so my mother was gone and my dad was at work. I work a part time job to help pay for school.

I was cut early from my job because it gets slow around new years and got home a bit after noon. When I got home I went upstairs and heard moaning from my youngest sisters room. I was concerned because she isn’t allowed to have boys over yet let alone when no one else is home. Her door was left completely ajar, clearly showing she wasn’t expecting anyone to be home. My older of sisters was still at a sleepover with friends from the night before. I snuck up and peaked in and was completely shocked to find my brother fully nude with my sister fully nude having sex. Neither were looking in my direction so I stood there and watched. I thought he might have forced himself on to her but it became obvious from what they were saying she not only was enjoying it but initiated it. He pulled out and came all over her ass and back. I left the room and pretended like I just got home. They stayed in their own rooms while I sat in the family room thinking. My other sister got home and I couldn’t hold back. I spoke with her quietly about what I saw. She then confessed she had had sex with him as well a number of times by choice, and recently too. She had known this was going on. At this point I went from horrified and confused to jealous and confused.

I texted my dad that I was going to visit my oldest brother last night. I drove there and waited for him to get off work. When he got there he invited me in and I told him about it. He laughed when I told him saying he was always suspicious but thought I was involved too, which I’m not, I only just discovered this. We chatted for awhile and he told me 3 years ago he once thought he heard moans from the middle sisters room once and my brothers were the only ones home at the time. I told him how I felt and he also felt the same way for years. For whatever reason by this point the confusion turned to arousal and he also felt the same way again. We decided to have jealous sex to get back at my siblings. Afterwards we got fast food and went back to his place. What proceeded was a night full of some of the hottest sex I’ve ever had and almost no sleep. I went on birth control 3 years ago so neither of us had any concerns in the world. By morning I was both covered and full of his cum so I showered and returned home. The middle sister confronted me as soon as I got home knowing what I likely did after our conversation. She knows but so far no one else does. This has been the wildest 24 hours ever and my mind is all over the place now.