Why is she doing this, is she using me?

Well I started talking to this girl about a week ago, she lives about 2 hours away and immidiatly I liked her. She told me she dated my old stepbrother (My mom divorced his father), she told me she pretty much hated him because he dumped her.

Somehow he found out we were talking and he started tellin me she was crazy and always texted him and I talked to some of his friends and his girlfriend and they said the same thing and that she even tried to kill herself. before this she said to tell her who I liked and she would do the same so I told her I kinda liked her and she really didnt say much.

She is always asking me to comment her pictures and stuff but she never really shows any affection she just says im her best friend. I went and looked on some of my old stepbrothers pictures and I saw a comment from her on there and it was a really friendly comment and she just commented it today though she says she hates him… Now I think that she is usning me to get him jealous because he told me she sent her a text telling him that I was her best friend. Now if I ask her if she is using me and shes not it might ruin what little I have with her..But if I dont then ill really get screwed..What should I do???