With my girlfriend's sister

The sister of my girlfriend asked us to stay in our apartment for about 10 days because their parents were going abroad for work. Around 19 years old at that time, she's slim, almost no breast, a single introvert but friendly with people she knows. I have an apartment in a small city, only one floor. beside our bedroom we have a big living room, she was supposed to sleep on the sofa. As the apartment is not that big, me and my girlfriend realized it would have been difficult to have sex as she was there. Looking back, I suspect she'd been attracted to me since a couple of years. I remember a couple of times, as we were alone together for a couple of minutes, she would open her legs to show me her panties under her skirt. Personally, I find her type very intriguing to me. Back to the story. After a couple of days where everything went smooth and normal, I found some sexual tension in the air between the two of us, as her way of sitting on the sofa was intentionally very sexy and our way of looking each other changed. As I was not used to have her around, I found myself with a constant arousal and wondering whether she masturbated in the apartment yet. I was so aroused that I even managed to find a way to be alone at home, just to open her trolley to search for her used panties. It was an amazing discovery to find 2-3 cotton panties with a wonderful scent of pussy, that my dick became fully erected in moments. I used them to go to the toilet and have a private time smelling them, have some solo session before putting them back  and check that her trolley was still in the same place as before. As the days were passing by, everything changed around day 4-5.  That night, I awoke by the sound of a fast rithmic rubbing of sheets and a sexual moaning coming from the living room:  she must have been so horny that she didn't bother to keep quite while masturbating, even though the doors in between the room were open! My girlfriend is a well known deep sleeper in the family. After a minute or two realizing what she's doing and that is natural for people to masturbate, I start to get very aroused by her moans, thinking about her hand between her legs and her pussy... I decided to masturbate and let her know what I was doing: I made some moans on purpose, and started slowly to rub my dick, sometimes by shaking a bit the sheets, in order to avoid any doubts to her but still checking whether my girlfriend was deep breathing.  She almost immediately stop, then after a while she resumed her work, this time with tiny soft moans. I was so excited that I continued my solo session, in a couple of minutes I had a massive orgasm while paying attention to not wake up my girlfriend. in the meantime she was still having small little moans of pleasure, soon followed by one clear moan. After a pause, I was so aroused as nothing similar in my life happened, that I much needed to touch myself again. She proceeded again to have another session as well. we both were well into this loop that lasted a couple of hours, I had 4 orgasms that night. The next morning I felt almost guilty for what happened. I also thought that maybe she would have been upset with me, so I avoided a bit her before breakfast, but when we made eye contact, she had an amazing smile and was in an extremely happy mode towards me. During the next nights this happened again, till she went back home, but the other nights her moans were clearly softer. Sometimes I would fall asleep and wake up after a nap, with her still working on herself. So we had a sort of "going to bed" routine, we wished each other "have a good night!" and after one hour or so, when my girlfriend was clearly and profoundly sleeping, we started our sessions for a couple of ours: she on the sofa, and me in the bedroom. One of the last night I dare to get up from the bed as someone who is going to the toilet. She clearly stopped, so I trickly made some sounds as if I was laying down on the bed, but actually going silently to go and see her in a position that was not visible both for my girlfriend and her. It was almost dark, the sofa was partially facing the door, after a while I saw her shape: she was supine and her right hand as moving silently but fast above the covers around her pussy. I was afraid to get caught so I went almost immediately back to bed. when she went back home, I asked her if she had been ok with the accomodation. She said "Yes, so well that I will come here again sooner or later!" She actually came back here again twice and more things went on, but that's another story... I hope you enyojed