Well, if not in a vulgar sense, 'huge cock' could potentially be a misnomer or a very strange way of referring to something large. Maybe it's a big rooster on the family farm that the stepmom has some sort of story related to. But this is a very odd and uncommonly used way to phrase it if not being inappropriate.
Definitely not. Such stories involve private and inappropriate information. Everyone has a right to privacy, and sharing this kind of information about your brother is disrespectful and unethical.