Has she been acting like this with you since a long time? I mean, since you both were children.
How were you together at that age?

Or is her attitude recent? Do you have an idea why she is like that with you? How do you act yourself when you see her? And have you tried to speak to her?

Like Shattered Sanity says, it's important for us to know a bit more about your life, present and past, to understand things better and be able to give advices.