I think she likes me but I don't like her in return. What to do?

[Jack]

It wasn't long after we left high school, and I was a bit down in the dumps because of it and posted it on twitter. We had never spoke about it, and she messaged me and asked me if I was alright.

It's 3/4 months since then, and she talks to me on Facebook everyday since then. And if I don't reply to a message of hers it's always she messages me asking why I'm not replying and such as.

But this is the bit which makes me think she likes me. I've always fancied my best mate's sister, but it isn't often I see the two. But I was with them near the end of July this year, and I was messaging her about how I liked this other girl. Not so long after, she posts these two tweets.

1. "I get jealous way too easy"
2. "That should be me. Not her.

The fact she tweeted those two tweets, and that she talks to me 24/7 makes me think she likes me. She also asks me every now and then if we would ever go further than friends, and if I say to her "Well, maybe not" she will go "Good" and will not admit she likes me so she might not either but I don't know.
Even now. She's stopped on at school for the next two years whereas I'm going to college for a fresh start. I start Monday 17th, she started Sixth form this Wednesday. But she's sneaking on her phone while she is meant to be studying just so she can talk to me. She says she is talking to others too but I don't know.

So do you think she likes me? And if she was to admit she liked me, how do I tell her I don't like her in a polite way? Help!
By the way, I don't know if those tweets were aimed at me, she claims they weren't but I don't know