My brothers are younger, so never really protective of me, but it was pretty easy. I forget how I told the one I was closest exactly...I just outright said it. My youngest, again, I forget how I told him. But that's a good thing, because that goes to show you how little of a deal it really was.

The one I'm closer to, L, was accepting pretty much immediately. The youngest, R, has been taking his time...once in a while he says hurtful things but all in all he keeps to himself and it doesn't seem to bother him at all. But he is really freaked out that I give myself needles, lol.
In my opinion, siblings tend to be not too bad, especially compared to parents. If he's protective, he may just worry about you and try to "convince" you not to do it because he doesn't want to see you experience discrimination.