You're likely to face several years of questions ! Two years ago when my kids (10 yr old daughter and 12 yr old son) asked me if I was gay - across a huge gym full of 200 kids at their day camp - it seemed like the time to bring them in on it.
They were remarkably clued in to what gay means. Over the weeks that followed I gave them lots of chances to talk it up with me and let me know if anything about it made them uncomfortable. They couldn't have been sweeter about letting me know that me being me is what matters.
Over time, there have been some signs that my son, who is now in the throws of early puberty, is thinking about this more three dimensionally. Like most kids, the root question is "What does this mean for me [i.e. him - my son]?". He quite often lets me know he's pretty sure he's straight. (He also lets me know that his attraction to some of his sisters' friends is attraction, not lust! Yeah, right ! :wink
As I am sure you will find out with your brother (and with others) coming out isn't so much an event as a process. For my kids and me the next step was letting them meet my bf. Then the first morning my son came into my bedroom when Jay was with me (just waking up, nothing even R-rated).
One thing I know my son struggles with is the way his peers use "gay" as a general term of disdain or ridicule - pretty much separated from its dictionary meaning. I think he's starting to get it. For now, it seems he's ok with knowing that it doesn't phase me.
For a long time I had dreaded coming out to my kids. In the end, like every other coming out I've made, it was undramatic and un-traumatic. And, once it's done, it feel great when the people you care about, know more of you.