How do I apologize to my childhood friend?

For context, we're practically the same age and she came over 2 days ago. We had been talking for an hour or so, when she asked me what I thought of her. I said I thought of her as a bro, possibly even a younger brother as we did some pretty dumb crap together. She went quiet, before she got a bit pissed. I realized that I shouldn't have said that and started apologizing, but then the next hour got extremely weird as she attempted to prove she was "feminine". It wasn't like her at all, and I felt extremely bad. How could I realistically apologize to her?