My girlfriend's sister [24F] has been staying with us for the summer while she waits for her new lease to kick in. It's been a week and I think she saw something she shouldn't have. I think she watched me [33M] creampie her sister [30F]...

It's been great having her stay with us. We love spending time with her and it's nice having someone else in the house. That said, my gf and I have been having a bit of a dry spell in the bedroom recently, so I was a little disappointed that having a guest stay over might continue that streak. To my surprise, my gf must have been missing our sex as much as I had because she cuddled right up next to me and took control one of the first nights her sister stayed over. It was kind of hot knowing her sister was in the house, but I knew we'd have to be quiet.

My gf seemed to have other plans because she started making more noise than usual. She sucked my cock better than I'd felt in a long time. She was gagging and massaging my balls and it was really getting me going. I flipped her on her stomach so her head was towards the door and I got behind her and started fucking her. I tried to keep it slow and quiet but my gf started throwing her ass back and was really fucking me.

I grabbed her by the hair and started to fuck her right back. We both started moaning more as the sex was honestly some of the best we've had together. She pushed her face into her pillow and started moaning that she was going to cum. I smacked her ass and told her to cum like the good little slut she is.

My gf was a puddle beneath me. I kept fucking her slow and hard building my cock to climax. Just then I looked towards the door and noticed it was slightly ajar. Outside the door was the unmistakable silhouette of my gf's sister. I should've stopped... But I didn't... I fucked my gf harder for a few more pumps, and came deep inside her. I fell forward on my gf and whispered what a good cumslut she was.

When I looked back towards the door, Sara was gone. I haven't been able to look her in the eye since. We keep having small awkward interactions but I don't dare bring up what happened. The worst part of it all is that my wife is going out of town for two days and it'll but just my gf's sister and me in the house.

The thing I can't get out of my head is I could swear she was touching herself when I saw her watching us...