Was gonna ditto Max's response, and then say something like, 'And add something like "Or were you that guy who had a really small one? Either way, don't remember it. So couldn't possibly have been all that great" ' cause I mean surefire way to bruise his ego, and knock him down a peg... And yes definitely he needs that.
But then you posted your brother's response to it... And well, I DEFINITELY want your brother hahaha. I'm DYING laughing. And he didn't out you either for not having a vagina yet either! So as annoying as siblings can be, you definitely have a keeper for a brother. Tell him he's earned himself a big fanclub here hahaha.