Just to keep you updated on how this ended, I f-closed her yesterday night, which I'm very happy about. To do a quick sum-up of what happened, I had to help her sister with some stuff and we had dinner together, her sister had to go for handball and we just had 5 minutes by ourselves. I decided to just take off to avoid excessive awkwardness, when I left she gave me a long hug, said I was sweet, and we had a long eye-contact, but rather than make-out, she grabbed my shoulders and turned me around towards the door, heh. I guess not very alpha at all to let myself be directed by her like that, but it was quite an intimate moment.
After this, she was on my mind a lot for the next couple of days, and I guess I knew I had to text her, so I did yesterday - she was really quite non-responsive to my texts, or at least very formal, but I guess that's her text-style. We ended up arranging to meet up at a bar, the two of us, a friend of hers, her sister and a friend of mine. She was acting totally different from her texts, held on to me when we hugged, held eye contact with me etc. Eventually, we ended up just the two of us as the rest took home, and were having a fun time talking and flirting more and more. I think I was pretty much behaving in a rather AFC-ish way. Well, not like buying her drinks and giving her compliments, but I wasn't at my best, I thought - I know only the very basics of PUA, I used some kino and said to her that we couldn't make out or something like that. I was afraid to go for the kiss because of the way we sat, which was good for talking, but bad for attempting the kiss. There's was lots of eye-contact and laughing, but as the bar was closing, I still hadn't gotten the k-close. She had said stuff like "Well, sometimes you gotta do what you feel like, and not worry too much about the consequences", while looking me quite deeply in the eyes.
In fact, I was so nervous about attempting anything that as we were walking, she asked me if I had a piano in my apartment, at which point I guess I finally picked up on something, told her to come and hear me play, so we went to my apartment, I played piano for her which impressed her quite a bit (I'm a pretty advanced classical pianist - I sometimes wonder how many girls I'd get if I was as good a guitarist as I am a pianist

), and I guess it was what you call LMR when she said "Well, I only came to hear you play piano so I guess I should go home now", to which I replied "Shut the fuck up and come sit in my couch", where I was sitting, and well, from here the rest was pie.
We weren't drunk, so neither really had any regrets today. She told me as she left, "it was stupid, but I don't regret it", the stupid part cause of her sister I guess, and we did a bit of kissing as she left. I've felt really good about it, it was all really quite intimate, and I had earlier developed a bit of oneitis for her. Sorry for the long post here, I guess I needed to get some stuff off my chest.
Thanks for your help, which made it all end happily.