In love with my best friends brother

Anna and I have been best friends for 8 years now. Her home is my home. My home is hers. We've seen each other grow up (shes 21, I'm 22) we have been through thick and thin.

Of course she knows my two siblings. And I know her three siblings. We're family.

Well. Her brother is the same age as me. I've always seen him as Anna's brother. Well. The night before her wedding he asked me out. I've always liked him. We have always gotten along really well. I said yes. My best friend even said if it doesn't work out, I'm still her sister no matter what. But we're both so happy. But a small part of me is worried about the potential awkwardness if it doesn't work. What do you think?