Best Friends Brother

So I don't have anyone else to talk to about this. I have had a huge crush on my Best Friends Brother for the past 10 years now. We did date for a few week about 2 years ago and my best friend was not okay with it. I started talking to him again this past Monday and we ended up having sex. He said he has wanted to do it for a long time now and that he still wants to be with me. I really want to be with him also. I talked to my friend about it and she said that she was okay with it as long as we are happy together, but as soon as I went to talk to him she got mad. What should I do? Any advise?