This happened the first time Mike came to visit me at my mum's place. We have quite a big house - both my sister (4 years younger than me) and I have our own rooms and we share a bathroom and my mum has her room on the other side of the house (with a much bigger bathroom - grr!) My parents split up when I was 10 so we haven't had a man in the house for quite a while.
Things went quite well for the first time Mike had met my mum and sis - there was the usual awkwardness that my friends say always comes with these situations but they seemed to get on. He was visiting me for a week, then had to get back to York to work. There isn't a great deal to do around where I live so we went for walks and I showed his around the places I grew up. I think I was a bit more reserved than usual as I knew that most people around would know me so there wasn't as much outdoors fondling as I would have liked (we did manage a few sessions and he did eat me behind the bikesheds at my old school - it had to be done :)
Mostly this meant that I was feelign quite horny when we went to bed and we had longer than usual sessions. The door for my room is on the opposite side of the hallway from my sister's room and our bathroom is between us (at the end of the hall) so at first I had been trying to be very quiet. On our third night (I think - not entirely sure) Mike was taking me from behind - his cock in my cunt and one of his thumbs jammed in my ass (I had come to love this position) and I was giving a bit more vocal encouragement to him than I realised at the time. I came fairly loudly (still not as loud as I can be). Mike had thought he heard someone on the other side of the door but I didn't find this out until later that evening.
He came over me and immediately went down on me and started licking my pussy - I had recovered from my orgasm but was still fairly sensitive so it took no time to get me going again. He kept encouraging me to tell him what to do and (as I knoew he liked it) I dutifully told him to lick my cunt, lick my ass, finger me, etc. He kept me going for quite a while, getting me close to cumming but not pushing me over the edge - at the same time he was jacking himself off.
Just as I was about to cum, he slid up my body, slid his cock into my cunt and started fucking me again. I was on the verge of cumming when he told me that he thought my sister was listening outside the door. I froze and wondered what to do but he kept fucking me and came inside me. The shock had certainly reduced my desier to cum, but he kept saying that if she was actually there, that she had already heard pretty much everything and may have heard me cum the time before.
He had moved sohe was lying beside me and was playing with my pussy and my resolve was beginning to weaken. He kept telling me to let her hear me cum again and saying that she was probably standing out there playing with her own cunt (I think that part was more for himself than me - and she was 16 so what he was saying was legal!) His fingers were still playing with me and the thought of someone hearing me cum (again) was bring me close to orgasm (even if the 'someone' was my little sister).
I gave in and asked him to eat me and came fairly quickly once his tounge touched my clit. I wasn't as loud this time but still audible. We spooned after that and a few minutes later I heard her bedroom door shut. I honestly couldn't decide if I had really enjoyed her hearing us or if I was going to be too ashamed to face her the next day.
She didn't say anything about it and I came to terms with it to the extent that we repeated the show the following nights. I still wasn't as loud as I am sometimes (only in an empty house!) but I did start using my full range of dirty talk. I think I justified it to myself by saying that she would know I had sex and would be able to talk to me about it when it was her turn (yes, I know that's a post-hoc justification).
On the last night that Mike was staying he suggested we up the stakes a little higher and left the door open just a little. Because of the way my room is arranged, we had to use some odd positions to make things visible from the door, but if she looked, she would have been able to see my little cunt being fucked and filled with cum.
I still don't know if she did actually look - we heard her door shut after we had finished (well, Mike did - he has better hearing than I do). We now put on a show (usually with the door shut) whenever Mike visits.
She has admitted to him that she has heard us having sex (because we haven't had a male in the house since she was 6, she looks to him like an older brother and he takes care of her - it's very sweet) but swore him not to tell me.