Found Out Something About My Girlfriend

So this happened when I slept over At my girlfriends place. Her and her brother Brad were roommates But after a year of dating I was use to him being there. He was cool and never bothered us and was gone most of the time. They were only a year apart so they had always been close so it made sense. We were comfortable to the point I was okay with fucking my girlfriend even when he was home and she assured me he never cared. So we had fucked that night before falling asleep at like 2 am. When I woke up randomly at 6am I realized I didn’t bring my phone charger in so I snuck out of bed to run to my car to go get it. I was outside for no longer than 5 minutes trying to find where my charger was in my car before I walked back inside the apartment. But when I did I heard a girl moaning “fuck fuck yes yes” and I thought DAMN Brad must have gotten lucky last night and was enjoying some morning fucking. I crept back past his room where the door was closed but then I stopped. I realized the moans weren’t coming from his room. I tip toed to my girlfriends room and peeked in, I was absolutely shocked to see my girlfriend with her face buried in her pillow with her ass straight up in the air getting her back blown out by HER BROTHER! I just sat there and watched trying to keep hidden while I watched my girlfriend get her brains fucked out by her boyfriend who I immediately thought to myself that his cock looked way bigger than me which was quickly confirmed to me when I heard Brad asking if he was bigger then her boyfriend which she confirmed with a “so much bigger, its soooo good”. I will admit even with hearing that and honestly a good amount of my girlfriend admitting I cant fuck her as well I was still rock hard watching this happen. It was like one of those i****t porno but I had a front row seat. After a solid 20 minutes of watching my girlfriend beautiful ass bounce on her own brothers dick I finally heard him say he was gonna cum. And just like whenever we have sex my girlfriend immediately pulled his cock out and swung around so she could put it in her mouth and get a mouthful of cum. After I could tell she swallowed his nut i crept back out of the apartment and started driving home not sure how to feel but was extremely turned on. My girlfriend called me 5 minutes later asking me where I went saying she just woke up lol. I just told her I was feeling sick and didn’t want to wake her. When I got home I jerked off for no more than 2 minutes before blowing a monster load. I think I’m going to hint to my girlfriend I’m okay with that stuff and try to get her to admit so I can try to watch again.