Does my friend's sister like me, and have I missed an opportunity?

So this is an experience that starts back in Year 10 and has carried on in phases throughout school until going to uni. It's very long and complicated but there's no other way I can explain it properly.

I fancied my best friend's sister, Sarah, who's just a year below me, but as I stand today, I really don't know what she thinks or thought of me. I've told my friend this and he was not only cool about it but even gave me tips on what I could do next to try and set things off.

When I first started fancying her, I got an impression she fancied me too. One of the first things that happened was when I was in Year 10 (and Year 9 for her) it was an assembly and I was at the front with someone and we had to throw out chocolates to some people, and she was sitting in the front row and mimed to me “give some to me” and so I did. I later asked her on Facebook if she enjoyed it and she said yes and thanked me. Starting with that, we used to talk on Facebook a few times and have really friendly conversations which I would always start.

Once at my friend's house for a campout, I was alone in the tent out in the garden, and I could have sworn I heard a young, feminine voice saying my name twice from inside the house (and she is the only other child). Later, when passing her inside, Sarah gave me this really friendly, wide smile. One time I even got a message from her account saying "I LOVEEEE YOU!". I thanked her for saying that, thinking she'd written it, but I didn't mention anything on how I also felt out of embarrassment. She responded saying that a friend had gone on her account and written it instead, but I found it odd why it was targeted specifically at me. Around this period, she also looked at me a few times when walking past her in school, and at one point she was coming down some stairs and I was standing at the bottom waiting to go to my lesson, and her eyes literally kept darting back and forth from me as she walked from the stairs but she wasn't showing any facial expressions. It was like she was trying to be inconspicuous about it.

In fact on the first birthday I had with Facebook, which was roughly around this period, she posted 'Happy Birthdaay!?' on my wall (if the morning after) which I gave a positive response to, but this was several years ago now. She hasn't done it for any others since.

However, as time went on we seemed to drift apart. Our conversations became shorter as her responses were more vague and brief, almost as if she didn’t really want to talk, and so I messaged her less and less, every time with the same types of response, until I stopped. She also stopped looking at me when we passed, at which point I thought she didn't like me back and I just left it as it was and moved on. I didn’t really think about it after that as we had totally zero contact for a long while, thinking she felt nothing towards me, but then some other odd stuff happened.

I've had a look at some of our older messages and the way the conversations went really varies. Sometimes I would say "Hey Sarah" and she would just say "Hello!" before I said something else, like asking what she was getting up to and she would respond (it was 50/50 on whether she would ask about me as well) but the conversations never really seemed to go beyond this. Other times, mainly in the most recent ones when I was in Year 13, I asked how she was finding sixth form and sent a kiss. She responded, this time addressing me by name, which she hadn't really done before, but didn't send a kiss and so I obviously didn't persist with kisses after that.

So one time in the Library, now in Year 13 and Sarah in Year 12, I was sitting at a computer on one side. Her friend was working at a computer on the opposite side and one away from me. Sarah then came in and started talking to her friend. After 20 seconds or so, for some reason the friend logged off and then they both moved round to my side, with Sarah sitting on the computer right next to me. What I found strange was that they didn't really need to move at all as there was a computer next to her friend on the other side. We said nothing, but it still seemed odd, and it never happened again either.

Then after several more months of nothing happening, I was standing outside her form room as my form was having a quiz with hers (I didn’t know it was her form at the time, though), and as she came up to the door and went in, she glanced at me and said "hi" to me, and I said “hi” back, and this was the first time she'd ever spoken to me in person.

A week or so later I followed her on Twitter, but nothing happened for a while whilst she remained continuously active online. In the meantime, we again had no contact for a few weeks until I went to another friend’s party. She was there with one of her guy friends (they are just friends as they’ve known each other a long time) and for most of the night she was hanging out with him, but at one point as I was talking to my friend, Sarah came up to talk to him and then whilst there she said “hi” to me and asked how I was. Despite her talking to me, the whole conversation for some reason felt really awkward between us and it quickly tailed off (but at least she made the effort to talk). I then just saw her and her guy friend hanging out at the party for the rest of the night and again thought there was no chance of her liking me, although she glanced at me a few times at the party since that encounter, which was a first after a long time.

Following on, several weeks after I initially followed her on Twitter, and a good week after the party, she followed me back. Thing is I don’t understand why it took her so long to do it if she was going to anyway, and why then?

Finally, after I'd left school I sent her a message just wishing her luck for her final school year and whatever she did afterward, and she sent back an enthusiastic short paragraph thanking me and briefly asking me what I was doing.
It read "Thank you Rob, that's so nice of you!! It's not late at all, my first exam I this morning so just in time 😂 which uni are you going to? Also good luck with everything as well".
This seemed the most chatty she'd been in a while. When I told her about the uni I would go to she said "Sounds fab, I'm sure you'll really enjoy it!". That's all that was said but it seemed to go well.

I got Instagram soon after and I followed her and this time she followed back straight away. I stayed over at my friend's days before I went to uni and saw her very briefly then and she said "hi" and possibly asked how I was, but that was it really. She didn't stick around long as she was doing something. She's liked the odd post I've put up on Instagram but since starting uni we'd not had any contact at all.

I came back for Summer and we still had no contact until toward the end when I found out that she was going to a party that I was. 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. Admittedly later on she spent the rest of the night getting off with another guy at the party, but this was drunken.

After this I have now been considering asking to meet up with her. Thing is, I would firstly have to message her asking her if she'd like to do that, and then of course make the effort to travel down and see her at her uni if she agrees to it. I just don't know how this effort would look for her, even if she likes me. I don't mind doing this in theory, but I wouldn't want it to look odd given the circumstances.

After all that, I know this is just speculation, but from this do you think she likes, or at one point liked me, and have I maybe now missed my chance? Or could there still be a chance to try? Thanks for any suggestions!