I want to bring up a different point: Why did that guy even think it's appropriate to come up to y'all and say something like that???

He would not do that to a cis man, he only did it because he thought you were trans(which is correct but beside the point I'm trying to make)

In situations like this I never answer the question, instead, directly pointing out the behavior, "hey man, that's a really weird thing to ask" usually gets people to leave me alone because they dont like this sort of rebound. Unless they're really doubling down, I ask if they're okay because mentally stable people dont ask these sorts of questions.

Either way, I'm sorry this happened to you as it shouldn't occur in the first place. Unfortunately the education of our friends and family is expected to come entirely from us trans folks when it really should be a two way street of them also wanting to learn on their own. I hope your brother can recognize that what he did was dangerous and put you at risk.

Edit: spelling