Actually it's not.
You have had a brother for most of your life if not all of it. You form your own identity around your family unit - I am the elder sister to a younger brother. Your family dynamics reflect that. All your memories and formative learning are coloured by this.
Then one day your sibling says "your relational identity doesn't matter to me. I'm going to throw a hand grenade into your life and expect you to go with it immediately without any transition phase for you to emotionally adjust too"
It might be a belief you just don't share because it is essentially an act of faith to believe you are a different sex and you may find the whole thing utterly sexist and homophobic. But hell since someone in your family believes now you must also join the one truth faith.
And if you don't do all the 'right things' some dickhead on the internet will be along to abuse you for being concerned about your brother. And that person probably hasn't had similar happen in their own core family. They might know someone trans but it's completely different when it's your family because of the collective and formative identity formation aspect.
People like this dont understand the impact. When you are asked in polite conversation 'do you have any brothers and sisters?' a simple question becomes a minefield. What someone is asking is a way to relate to you. They want to know if you have a sister like them and how your relationship was growing up and whether you go on etc etc. And that just it. It's NOT the same if you have a brother who transitioned. You can't relate. And then there's the people who don't want to get into the politics of it and are weary about what your politics are.
Then there's the trope about 'but they are just the same person' when they are literally asking you to treat them as someone completely different. People who are not closely just do not get it in the same way.
The OP clearly cares about her brother and is concerned for his health. Which she rightly should, because not only are they taking drugs which are harmful to males and have significant side effects but he's buying them online so he has no idea if what he is buying is legit or cut with god knows what else.
But yeah you crack on with your virtue signalling self righteousness. I'm sure it makes you feel proud and big and clever.
You have absolutely no bloody idea.