Brax is the Doctor's Brother? Where does that come from? I only know him from Gallifrey Adventures (never listened to The Benny Summerfield Audios yet nor do I know if he's in any other BF Audio Plays)
It's heavily, heavily implied but, as best I can remember, never outright stated. The Benny audio Death to the Daleks
reveals that Benny's Irving Braxiatel is a Time Lord and has a TARDIS in the Braxiatel collection (when it dematerializes in the audio, it's a jaw-dropping moment), and Cavan Scott and Mark Wright's "...Be Forgot" in Short Trips: A Christmas Treasury
has the eighth Doctor visiting Benny because he's on his way to give Brax a Christmas present.