how old are your parents? I have seen waitresses say this sort if thing to include my father, might have been to improve tips. possible that it was just your brother not understanding and saying something embarrassing. don't worry to much about it.

I was definitely mistaken for a boy multiple times as a preteen! I had short hair and wore baggy punk clothes so that's understandable but once I was at a splash pad, in a bikini, had started to develop a little bit and some lady yelled at me for being a boy in a bikini and that she would call the police🙃 I still think about that sometimes and just wonder wtf lol, maybe I just had a really boyish face and the short hair (that I was growing out) didnt help lol.

at least she thought you looked nice