So, I know her for like 2 year or so and I love her since. And she knows I do (thanks to my sister). Never looked like she loved me. But, one day when I logged in on Facebook she used one of those pesky FB apps to make a question, if I knew all her secrets. Another day she asked me if I heard a Rihanna song (Diamonds I think) and if I didn't, I should. And she knows I ONLY like Metallica, Pantera, and so on. I may be speculating because I really like her. Is this one of those girls "mixed messages" or is she saying that she likes me?

Who knows? We are total strangers. She could be just being friendly. She could be being coy. OR she could be wondering when you are going to stop hiding behind your sister. Sure, you might get shot down and that will be painful. But if you don't show a little eagerness in person with her, you could be missing out on the love of your life.

It's hard to say. She may just be being friendly. If you have liked her for two years you either need to approach her or forget about her.She is not going to read your mind.
A closed mouth does not get fed.

There is nothing mixed about starting a friendly conversation, just don't get carried away with your own feelings and see more than what it is.

I doubt she has the same level of feelings you have, but that's why you talk friendly as she does. No risk, no reward.