I agree about sounding him out, but do be aware that there's a lot of transphobia in certain sections of the gay community. As a member of both communities, it saddens me greatly - but it is the reality a lot of us face.
At a recent trans pride event I got into an argument with a cis gay guy who didn't realise I'm trans (I pass 100%) and he kept moaning about how trans people are just a bunch of whiny extra-special snowflakes who are only doing it for attention. Sadly, this isn't an isolated incident. I tried explaining to him that the things he was saying about trans people are the same things people were saying about gay people back in the 1980s, but he was too young & too drunk for that to sink in.
I hope your brother is more understanding, but please be aware that just because someone is LGB, doesn't necessarily mean they accept the T. It might be helpful to prepare some information for him, just in case you need it. But as long as he loves you, he'll come round no matter what.
It might be better to just play it straight (pardon the pun): sit him down & tell him you've been struggling with your gender, have decided to transition & need his continued love, support and understanding.