There are two constants in Katara's life. One is waterbending, and the flow of life that accompanies it. The feeling of power, so recently exploded into frightening and exhilarating extremes, is as much a part of her as breathing.
The other constant is Sokka, in all his exasperating glory. Sokka would never take her seriously, Sokka would never let her play with his weapons, Sokka would always be lazy, Sokka would always eat more than his fair share of the food, and Sokka would always, always be there.
Mother had always been there, but she was no longer a constant. Father might come back and take up his role as a constant, but there had been painful years where he wasn't, and Katara would always remember the time when he was not there.
Sokka would take whatever she could throw at him, much the same way water would continue flowing even if you kicked it until you collapsed. Any insult she threw at him would be met with an even more insulting reply, and every time she beat a goal he would already be ten steps ahead finishing the next one, looking over his shoulder and laughing at her.
There were few things more frustrating than an elder brother who didn't take you seriously, and Sokka reveled in teasing Katara to the point where she thought she hated him. But one moment she'd be glaring up at him, an instant away from stamping her feet in anger, and the next they would be back-to-back, surrounded by danger. She would charge forward with the self-confidence that comes from knowing that your back will always be protected. Love and hate were irrelevant, to those who were forever connected.
She saw nothing wrong in depending on Sokka so much, because that was the way it had always been. He might have been sub-par in forest hunting, and in the icy countries he might not know how to waterbend the landscape around them, but he was the ground beneath her feet, and the line that kept her tied to reality. He would grumble, and complain, and sometimes sulk when she wanted to go save the world, but he would be by her side anyway, eyeing off the thugs and bringing his blade down on any who strayed too close. He would tug at her hair, and shout at the world around them, and sleep too much, and he would always be there; a reminder that no matter how far Katara traveled, she would always, always have a way home, and a family waiting there for her.
And with that, she was content.