I was speaking from experience. My brother has outed me to people in a couple of ways (not always related to being trans), for example, but its never been a problem. In terms of being trans, I think he's only mentioned it to other trans people (maybe only one? Idk) I don't really care - its not exactly something I intended to keep secret. I just don't like to bring it up, so if its already done, that's easier for me.

He did ask for permission before saying anything to our cousin (who is also our roomate), but my cousin forgot and somehow never picked up on it despite many conversations between my brother and I that only make sense in the context of me being trans, changing to they/them pronouns on twitch, and some exploration with presentation (like wearing a dress around the apartment or cosplaying as a girl character). For the longest time, I suspected he didn't think of me as trans because he'd sometimes reference another person we knew as an example of a trans person but for some reason never mentioned me in those contexts (given I'm not really out, it makes some sense though not to mention it). I mentioned I might try to get HRT soon like two weeks before I actually started and he was still a bit surprised when I did the thing I said I would do (I don't think he knew what HRT was). So... I think he knows I'm trans now? Guess technically I didn't say I was though, so maybe he thinks I'm just a cis guy taking E?