Hooking up with best friend's brother!?

So I hooked up with my best friends brother..And now me and him don't know where are relationship stands...we didn't have sex though. She doesn't know but I feel like a terrible friend, but I don't not want to tell her cause its wrong, but then I can't tell her because everything will be messed up..Advice?
Hooking up with best friend's brother!?
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