Kanna was for the longest time the only family Sokka and Katara had. With their mother gone, and their father off fighting the war against the Fire Nation, it was Kanna who became their rock.
Kanna herself had quite a history as we would find out. She was born in the Northern Water Tribe. As was custom, a young man named Pakku (who would become Aang and Katara's Waterbending teacher) carved out a necklace for Kanna to become bethroved to her. Kanna was very resentful of the ways women were treated in the Northern Water Tribe, and thus fled to the south.
She was there when the Fire Nation began raiding the South Pole and taking Waterbenders, including her friend Hama. Kanna was one of the last ones to remain.
When Aang was found, it was Kanna who encouraged Sokka and Katara to not only find him after he was taken by Zuko, but to go with him and save the world once he was safe. For as she noted, it was important they followed their hearts. As that was what she did.
Ironically enough, she and Pakku did get married later on after Aang and Katara's journey to the North Pole.