Not All Twins Bend the Same Element (Avatar...
Katara and her twin sister Kayta are benders from the Southern Water Tribe. However they do not bend the same element.
Katara is a waterbender, but Kayta is a firebender. That's not the only thing that is different about Kayta though. She has two different colored eyes, silver and amber, red hair, and she did not inherit the olive skin that her whole family has.
Kayta and her sister are extremely different, but they both want the same thing. To learn how to bend properly.
Maybe helping the Avatar will help Kayta understand why she is so different, and how to control such a difficult element.