My best friend is dating my cousin and its so weird now?

We have known each other for 6years now. My cousin and my bf have been together for a year now. At first I was so excited about having my bf in our family but now I'm not too happy about it. So many things have changed. I feel like she isn't the same friend or even person. The only time we talk now is when it's about my family gossip. It really others me. Plus she had the nerve to go off on my family members about a certain subject. I don't know if she felt comfortable because she's our friend but it really irritated me. When we talk now it's very awkward and we don't talk much now. I haven't told her how I feel but I think she is seeing the change too. Need some advice on how to tell her how I feel without making the friendship worse.