I wouldn't take it so hard, Michael. Love is all that really matters and I am sure your brother loves you and so does your mother. We can't be everything to them that they expect us to be. I am not everything I expected myself to be. When it comes to transsexuality it is difficult enough to explain it to someone who really wants to understand let alone explain it to people in general.
A former highschool girlfriend I am now getting reacquainted with on a friendship level is being very supportive of my transgenderism and is trying to learn all she can about it. She bought me the movie, Transamerica, which I had already seen but she hadn't and so we are in the middle of watching it. Meaning, I talked so much through it pausing every few minutes to explain things that we ran out of time and so she will come again to watch the rest of it. The reason I am bringing this up is because there is a part in it where Bree is sitting in a restaurant and a young girl asks her if she is a boy or a girl. That freaks Bree out. The comparative part comes later when Bree and her son are at the house of an mtf friend who is having a TG party. Bree comments about how one women is so not passable. Her friend tells her she is a GG from Mary Kay. The point being, we all do it. The important part is what we do with that information, acceptance or non-acceptance.
When someone asks a question or makes a statement about someone who is transgender or transgenderism in general we should try to answer the question without judgement. Being able to talk about it is a really big thing. It is more important than we may think. I am elated when I can talk to someone about it.
I wouldn't be upset with your brother. He felt free enough to talk to you about it in the first place. Correct his thinking, discuss things if need be but leave the door open for more questions or comments so that you can continue to educate him. If you close that door out of anger or disappointment he will stop talking to you about it.
I hope this helps.
hugs,
Bonnie