The Avatar~ The Avatar State
Aang learned his native element, air, from a friend of the previous avatar(Gyatso) then his final element, fire, from a descendant of the previous avatar(Zuko, great-grandson). Korra learned her native element, water, from a friend of the previous avatar(Katara) and then her final element from a descendant of the previous avatar(Tenzin, son). So basically, avatars learning their native element from a friend of the last avatar and their final from a descendant.
Requested by japaneesee