I think Alison's correct: "ask your brother what he wants." Of course, you have to feel comfortable about it, too.
I'm somewhat surprised that your brother would EXPECT you to have explained the situation. I certainly never wanted that and, to this day, don't want that. I moved to another city to avoid such "outings."
Recently, I went with a friend to a get-together in another state, California. I had a nice evening talking with everyone. I particularly enjoyed talking with the host of the party. Transsexualism never came up in the conversation. Imagine my surprise when I found out from my friend that she had, before the party, discussed my male past with that host. Many post-op transsexuals like me want to forget about our past. When people know of my male past, it brings up, in my mind, a discomfort: "What are they thinking and not saying?" It means that, when I'm talking about the presidential race or the environment, they may be nodding their heads and talking about the same thing but, in the back of their minds, they may be thinking, "hmm, this lady was a guy, huh?" Without the tip-off, I'm 99% passable. Even if they agree, in their heads, that I look and sound like a woman, who needs it?
I'm not on this earth to act as an example of transsexualism. Maybe, in less bigotted times, I'll feel differently. But when gays are still fighting for a simple thing like marriage, I know it's going to be a long time before acceptance becomes a NORMAL thing for people like me.
My ex never talked to others, "outing" me. She felt that "good news travels fast -- especially, anything to do with sex." When I transitioned, the pychologists, as part of the program, demand that I be "honest" with co-workers and friends. I outed myself so many times. It lead to long discussions which, as you say, "ends up dominating the out time together." It can get tiring after awhile. You long for normal conversations. I'm guessing that your brother is new to transitioning and, presuming from what you say, likes to talk about it. He too, like me, may tire of it.
At times, I fight for TS causes. Most of the time, though, I want to live as any average middle-aged woman would, in peace.
Teri Anne
