This should be her choice who she wants if you got to know from her about her relationship with your brother you can directly talk to her about this. If you got to know from somewhere else then you should wait for her to disclose it to anyone or you. Understand her and your brother, you can’t be jealous or insecure about this relationship because this is understandable you can be with a person who is genuinely happy with you. 
Your best friend might want to tell you but because it’s your brother she is with maybe that’s the reason she isn’t sure. Do not hurt your feelings or others.