I do not plan on giving into his wishes and wearing a dress, no matter what he wanted. Yes, he is my brother, but that alone does not mean he has my respect. My love, yes, but not respect. He still has not earned that.
There is no way I am going to wear something I am not comfortable with. Even if it weren't a dress, but something else, if I was not comfortable with the idea of it, then I would not do it, and him insisting otherwise is just rude and inconsiderate. It may be his big day, but this is something I cannot do, not for him.
If he does not want me to serve because of this, then I won't, and I will do as suggested and sit with my mother, dressed in proper attire (of course).
As for "I'll wear one if you do." That wouldn't work because he's the kind of guy that would take me up on that just because he can.
Yeah, there's still a year before this even goes down, but he's stressing about it now which is why this came up. In the time until the actual wedding, a lot will happen that will hopefully assist him in accepting me as I am, so, right now, I'm not really worried about it. (This just irritated me more than anything else.)
If it comes up again, I will make an attempt to speak with him, and explain things a bit better, but for now, it probably is just better left alone. There is still a lot of time, and I'll see if he comes around.
Thanks a lot for all the responses, everyone. I really appreciate it; all of them helped.