Hi, your brother has a lot to deal with, first he may be in his eyes not yours, loosing a brother(strong bond thing), and a role model, for which he will have a lot of confusion and resentment. If his peers know about you then he is also going to be getting an extreem lot of pressure from them, pressure you cannot imagine, He will then have to support his own "male persona" in a strong way "an over compensation' to maintain his own integrity and position in his own peer circles, this would mainly be in strong rejection of your transition, Over time and peer change along with a bit more maturity this will diminish, maybe not completely because of the first point, but he will 'lighten' up over time but give him heaps of it....
If you love him as a brother and he knows this, things should work out eventually.