When it came to explaining that I'm FTM to my two younger brothers (at the time they were 4 and 8 yrs old), my dad and stepmom took care of it. Ever since my brothers can really remember, I've dressed like I do now (Like a guy) so that has never really been an issue, but when it came to starting T, my parents explained it as something along the lines of "Lex (which is what my brothers call me) is going to be taking some medicine that's going to make (unfortunately they all still use female pronouns with me) her feel better and happier. Lex is going to start sounding and looking more like a boy, but it's ok."
My brothers have never really asked anything, but the older of the two did say something like:
Brother "So, I heard you're taking some medicine now, like a shot?"
Me: "Yep, it's going to make me feel better"
Brother: "Does it hurt?"
Me: "No, not really"
Brother: "Oh, and your voice is going to get different?"
Me: "Yep, that's the plan"
Brother: "Oh, ok" He smiled and went on to play his DS.
So really, I haven't had a problem with it, and they haven't seemed to either. They treat me like an older brother, but still use female pronouns for the most part , but I figure that'll change in time. =]
I hope I helped at least a little.
Good luck!