Why was my friend's sister suddenly really friendly with me?

I've had a crush on my friend's sister for a few years now, who's just a year below me. My friend (her brother) is the same year as me. I've told him about how I feel about her and he was not only completely cool about it but he even gave me advice on what I should do next in terms of making a first move, so I'm not at all concerned about the social aspect of it.

I've not really had what I would call a proper friendship with her though, which isn't a problem. During the school period when I first really started liking her, the only interactions we really had were smiles in passing or looking at each other. I'd messaged her a few times about how she was doing at school but that was about as far as it went.

When we were both in sixth form, she then started saying "hi" to me if we were around each other and asking how I was, but this was literally as much as we did, and it also sometimes felt a bit awkward as well. This was the case at one party and we didn't really interact with each other for the rest of the night afterward.

I'd written all this up in much more detail on another online discussion before and it got a few responses, and the general consensus that I got was that she might have liked me before sixth form but has moved on now, or was still interested to at least see how I would respond around her now, so I thought possibilities were still open.

So last week I knew that she was going to a party that I was, so knowing I would most likely see her around I casually sent her a message (if a bit late) asking about her A-Level results as a basis of potential conversation at the party. All she said was "Alright, I got Bcc" and nothing else, so by then I really thought that she wasn't interested and that at the party we would pretty much stay out of each other's way.

I went to the party a few days later, and the first time I saw her she was standing by the back door talking to a few people. I casually walked past to go inside and she turned to me and said "Hi! I haven't seen you in ages!" and gave me a hug (we had never even come close to this before), and then asked me how I was getting on. Another friend came over after this and we all talked about her brother, who is currently away. When this ended, we naturally just moved onto other people and so I walked back into the house.

Later on, she and a friend she'd brought were dancing in the living room. I was just casually wandering around the area at the time, and she then just said to me "come and dance with us!" and so I did, and she showed me what moves I should do, and so I did. She also introduced me to her friend who I'd never met before and she gave me a hug as well, and then we just danced together for a few minutes afterwards. She constantly interacted with me during this and reacted positively to some moves I did.

What I just found odd about it all was how at the party she treated me as if we'd been good friends for a while, when this really wasn't the case and I genuinely wasn't expecting much interaction with her at all. After this I have now been considering asking to meet up with her when I go back in a couple of weeks (I don't mind potential travel arrangements).

I could be really overthinking this but it just seemed odd how suddenly she treated me like that. What do you guys think?

I know you want people to come in here saying she likes you , ...but really there was no concrete behavior from her that you've written that would greatly imply she fancied you