Was This My First Threesome?

Go to my profile below then find "My story" which explains how I went from experimental play to actual sex with my sister to understand this story better.

I was 19 making my sister 15. Nobody was home except us and our large pet male Samoyed called Sam. It was one of those days when we knew that nobody would be home for hours. We had already had sex once, and we were into our second sex session for the day. I had got my sister to hop on top of me and we were fucking facing each other, as in she was on top.

Sam wasn't a small guy and had quite a large cock that was also thick. He was always horny, and often inappropriate with almost anyone in the house. If anybody was silly enough to get on their hands and knees around him, he would be mounting them if they weren't careful. He had no shame either and would masturbate himself in front of whoever, he didn't care who watched. If anyone wanted to jerk him off, he was keen, and we had got him off several times that way. Even with my sisters friends joining in.

So while my sister was on top of me, Sam randomly wanted to join in. He was already hard with his massive cock out, and climbed onto my sisters back and tried to fuck her from behind. She almost screamed and jumped off me while pushing him away. His hips bucking automatically in the air as this happens. Sam! She yelled as I start laughing. I asked what did he do (knowing full well what happened), and she told me that he tried to fuck her and she felt his cock on her bum cheek. I said that she should let him. She said, no way. I said, in your pussy. She said what if he got stuck in there? We had both seen him with his girl and they always get stuck for up to half an hour. I said that we have time and he won't be stuck for long. I told her that it would probably feel feel awesome.

Anyway, she eventually agreed but only if I let Sam fuck fuck my ass first. I had been secretly bi for ages and the only one I trusted to know was whoever was involved, and my sister. So she knew how I liked being fucked up the ass. I told her that if she promised to do it, I would as well, but she has to go first otherwise his cock will be going from my ass to her pussy. Ewww! Was her response. I said, I guess you're first then.

She agreed and next we're discussing how she should do it. She wanted her to fuck her from behind, I wanted to see if he would fuck her in missionary position. I told her it would be less rough with her on her back and I would help guide his cock into her. She agreed so that's what we tried. He wasn't into it, so we both help to get him nice and hard. It wasn't long and he was suddenly interested. She put her pussy to his face and then while laying back, we encourage him on top. It was clumsy and we had several tries before he eventually bucked his hips, thrusting towards her open pussy. He rammed more than half his cock inside her with a jack hammer action, he fuck his big cock in and out of her about 10 times in about 5 seconds before hopping off her. She that was fast, and now it's my turn. I told her no, that she didn't even get fucked properly. I said that he has to get stuck for it to count. There was no argument, just her trying to get him back into her pussy. He got back in another few times, but it was sort of a repeat. She did seem to like it more and said it wasn't hurting like before. She then said that maybe it would work from behind as long as it didn't go up her ass. We put cushions on the floor, got him hard again and she got on her hands and knees, he mounted her and rammed her so hard she screamed in pain, as he got the knot in within about ten fast pumps. It happened so fast that I didn't realise that he was even inside her because he pushed her so far forward. She's practically screaming that it's stuck. I asked if she's sure, she started hitting me. Sam panicked as he hates anyone hitting anyone, so he went to run off, and yanks her to the ground by her pussy. Yelling in pain as they both fall sideways. I told her to just relax and it will be over faster. She told me he was so deep and stretching her pussy. Also that despite it hurting, it felt good and she could feel him throbbing inside her while he was pumping her full of cum. After a couple of minutes she was really enjoying the feeling and began a fucking motion, as she tried to add movement to Sam again. He just laid there almost falling asleep. He did look happy though. After about twenty minutes, his cock began to very slowly come out. Then suddenly his cock almost flew out of her followed by a heap of his cum. My sister was shaking from an intense orgasm he had given her on his way out.
This was probably the most kinky thing I has seen or done still. We both enjoyed it intensely, so it was repeated on other occasions.
It was about an hour later before I started with Sam, but that's going to be another story. Stay tuned...