What does her behavior mean ? What should I do ?

I like a girl very much. She is 20 years old. We used to live in the same hostel for some weeks. I don't know if she really understood i like her. Probably she does from my way. We went for a coffee for some times together with some friends. After that she left and recently came back. I am now living in another place and saw her on the street. She was very kind. We got our phones and i text her sms telling that i really am glad to meet her. And she didn't text me back. Then I found her Facebook and wrote to her. After some messages i asked her to go out for a coffee and she didn't reply at all. But after some more hours she added me to her Facebook. I know her elder sister and she used to tell me that "my sister is coming back soon" with a smile on her face indicating that she knows i like her sister. Until these I was thinking she also likes me. But she is very unresponsive. I stopped writing to her at all. I am afraid of over attention. I really like her. What should I do?