My gf's sister (kelly) who is 23 always flirts and has a laugh and a joke with me, she works about 5 mins from my house, I work from home. She comes over sometimes on her lunch-break just to for a coffee and a chat. One day she came over, I had just got out the shower and only had a towel around my waist, I opened the door an let her in, as I was walking towards the kitchen to put the kettle on she pulled my towel and it dropped. I turned around laughing and she was too, then she looked down and saw me naked. She winked at me and said "you might aswell leave your towel off now." I told her to behave herself and wrapped the towel around waist again, the idea of her seeing me naked got me aroused and I started to go hard under the towel. She noticed and said "you get excited rather easily" I told her that it turned me on to know she had seen my dick. She said "in that case, why don't you let have a closer look" I told her it would be wrong but it was something that I'd fantasised about for years. She got on her knees infront of me and pulled the towel apart, my dick was sticking out, she looked up at me and said "I would love to taste it, will you let me?". By this point my dick was real hard and I had pre-cum a little. I knew it was wrong but I just couldn't resist. I didn't say anything I just grabbed her by the hair and put my dick in her mouth. She sucked me off real slow but sucked it hard, it felt so intense that I came in her mouth within 5 minutes. Kelly got up off her knees and gave me a passionate kiss and said "if you don't want this to happen again, next time make sure you wear clothes when you open the front door" I told her that it can never happen again. She told me that she would only come over when my gf was home to make sure it didn't happen again. Over the next few weeks I started missing her and the jokes we shared when it was just us so I sent her a message asking her to come over sometime soon. She didn't respond. One evening when my gf and kelly were over we started watching a film, I found it boring so I decided to go to bed. My gf joined me about half an hour later and left kelly downstairs. We started getting intimate and kissing, we started 69ing as I was going down on her I looked up and the door was half open, I saw her sis standing there naked fingering herself while she watched us. My gf couldn't see her as she was underneath me with my dick in her mouth. I was so turned on by her sis masturbating and my gf sucking me off I came into my gfs mouth. Kelly saw me orgasm and let out a moan. At that point my gf looked round and saw her in the doorway, she pushed me off her and sat up. Kelly just froze, I acted as if I didn't notice her there at any point. My gf said to kelly "what the hell are you doing?". Kelly said she heard us all the time and was turned on by the thought of me fucking her sister and wanted to watch but didn't know how to ask without being told she was weird and ruining their relationship. My gf looked at me for my reaction, I just smiled and said "who wouldn't want to watch us making love". My gf asked kelly to come in and sit on the bed, she said to kelly "if you want to watch us, then you have to let us watch you". This was amazing for me, it was like a dream come true. Then my gf told kelly "you can watch as much and as often as you like, just keep your hands off us". I was a little disaPpointed but I knew that it was still an amazing arrangement. At that point kelly lay down on the bed next to us and started fingering herself. I was hard again watching her so I got my gf on all fours and fucked her while watching kelly finger herself. My gf loved me cumming over her, just as I was about to orgasm I pulled out and came over her ass and a gave a small load over kelly, my gf turned around and smiled kelly orgasmed soon after. We lay in bed after for while, not much was said, I wasn't allowed in the middle... The next morning everyone got up and went to work... It wasn't a regular occurrence after that but it did happen now and again when kelly was over but she rarely stayed In the bed until the morning. I want to fuck kelly so much but I don't want to ruin anything, I think my gf may let me at some point, maybe I'm asking for too much, I imagine its kelly sucking me off when my gf does it...