Well, I don't have any biological brothers (*Cough* Legally speaking, I have a half brother...but to me, he doesn't count) But anyway, I don't know what it would have been like to grow up with brothers, but I can say that my father was my primary parent in my life and that when I was growing up, his opinion was all that ever mattered to me - Since he was my role-model and main male figure in my life. Now that I'm older and now that I've come out as being trans, there was a point where he compared me to how he was at my age, "Well, I didn't wear girl pants," and "I knew how to throw a baseball correctly when I was seven," and so on and so forth, so...in a way, it was like dealing with a father and a brother all in one. At the beginning, he kept giving me reasons to justify 'why' I wasn't trans. So I guess, maybe I can relate to you in that way. Either way, he sort of got out of that and it's made it easier.
It sounds like your mother is still adjusting.
Curious. Were you the only female born child she's had? It may be that she's scared that she's 'losing' her baby girl and doesn't realize that you've always been a man inside. You could try pointing out more feminine bio-men to her to help her realize that men come with all kinds of personalities. I don't know what your sexuality is, but if you do that, I would try to avoid feminine gay men, as she'd likely do what my mother does and say, "Well, they're gay, so they're supposed to be girly," And as most of the trans community knows (And others don't) sexuality and gender identity aren't the same.
Either way, good luck, man!