I'm dating my friends brother and now she's acting strange?

All right people help me out with this one. I have been friends with this girl for about 3 years. We've always been close and comfortable sharing everything with each other. We met in college and are now both 22 years old. However, about 3 months ago, her older brother came back to visit and when he did, asked me out on a date. (I should point out that he lives in NYC...and we both love in Washington State. Yeah...) At first everything was cool, but they are very close...practically best friends. I know this and respect it and don't interfere with they're relationship but lately she's been distancing herself. I call and she doesn't respond to my messages. I txt and she barely txts back. I posted on her Facebook and she didn't even acknowledge it. I'm really starting to worry since I really do care about her too. As far as I know, he still keeps in close contact with her. But him and I have also gotten very close, despite the distance. We talk every day. Skype, text, you name it, and really care for each other a lot. I believe that she might be feeling replaced? I want to talk to her about this but she doesn't want to talk! I don't want this relationship to get in the way of our friendship, but I'm also afraid that she might try to get in between my relationship with her brother. What am I supposed to do?!