Niiice, I totally forgot Sokka and Katara's family were from the northern originally.
Jeff Palmer is and will be the greatest source for all things about the trilogy's production.
I just don't get the logic that the South tribe and the North Tribe look substantially different . But then again ,it really isn't that hard to explain with Mendel's method. Wasn't Katara's mother from the Southern Tribe? That alone would have made Sokka and her biracial and thus have exterior Inuit features ,but the opposite can be true . For example , many cases of biracialism happen that the children look more like the other parent . They probably just have inherited their father's white genes and share some features with their mother.
Zuko since his nation is Middle Eastern/Indian as well as of Mediterranean stock will most likely have a lineage from his Mediterranean mother and Indian father , which makes him biracial.
Aang, as we know by the information from this update ,is from an amalgam of racial gene pools. Perhaps that's why Firebending and Airbending are very similar to each other, but it also suggests Airbenders can be anyone who want to get away from the bustle of normal life . Thus, the environment is the ultimate deciding factor , or a very influencing one at that, that determines which person can bend what.
I'm a science wiz, don't kill me.:sweat: