I think it was both at Comic Con last year and somewhere in the Welcome to Republic City game at Nick.com (superfans, feel free to correct me about the Aang family tree origins).

Luckily it’s out there, so googling “Aang Family Tree” returns it mostly without fan-spoilers (like the bottom second from the right picture which shows Bumi as Amon which we’re pretty sure is fake).

Enjoy.

-Da7e-