So after mom dies, Sokka has a meeting with himself, trying to see what he can do. He really can't help father, whose grief is always private. Other than that first hug after he came back to find their mother dead when he cried with them, father hasn't been sad in front of them. He's there when Katara has nightmares, and he has tried - and failed - to learn to cook with Gran, and he always messes Sokka's hair when he asks if he's fine, but nothing else.
So Sokka thinks, he can't help him yet. Once he's accepted as a warrior helping dad will be easier, but for now... for now, he thinks, the biggest help would be making Katara smile again, something she hasn't done not even once since mom died.
So he gets all these huge ideas about how to make his little sister stop crying - and he's sure most of them would've worked - but since he's a little out of proper resources, he decides that the best option would be taking her to see the Northern Lights from his secret spot, the one he hasn't shown to anyone ever. Even mom knew about it, and Sokka never actually took her there, which... okay, he's not going to dwell on that.
It would've been easier if Katara actually wanted to do something. She does the things that Gran asks her too but the moment she's done them she turns towards her furs and lays down in silence: she hasn't even soaked him with her stupid Waterbending powers once since mom died. But Sokka is still taller than her, thank you very much, and her older brother which should mean that he's the lord and master of her, really, so he grabs her hand and pulls and pulls.
"Lemme go!" Katara says, pulling at her arm.
"But we're almost there!" He says. The lights haven't started yet, and their mother loved the night rainbows so much. It's going to be worthy Gran pulling at his ears for breaking curfew if Katara sees them and feels better.
"I don't wanna go!" Katara yells at him, pulling at her arm strong enough that she manages to break free.
She also makes Sokka lose his balance so he's toppling down the snow hill. He stays there, counting till ten (billions) because he's a warrior and warriors do not hit girls, not even bratty and ungrateful little sisters who-
"Sokka!" Katara screams again, but this time scared, and Sokka keeps his eyes closed, still trying to remind himself that he does love his sister, really when Katara slides down to his side, and she seems about to cry again. "I'm sorry, I didn't mean to, are you alright?"
And just like that, his anger goes away. "Hey, sis, c'mon. I'm a warrior! Something like that's not gonna hurt me. And look!"
He points towards the sky. Katara's mouth forms a small 'o' and her tears glisten a bit with the colors of the lights from the sky. Sokka sits down besides her, one eye on the lights and one on his sister. And it might not be the best place ever to watch the night rainbows as his secret place would have been, but when Katara smiles in delight, he has to admit that it's not a bad second place at all.