Generally at a gig, and especially in your situation, the people watching want you to do well, they're on your side. Remember this.
Don't worry about little mistakes, just keep the music flowing. If you really get lost, drop out for a bar, regroup your thoughts and come back in where appropriate. As long as the band keep flowing nobody will really notice.
Try to enjoy it (as much as possible given the circumstances) and I'm sure you'll make everyone, especially your brother, proud.