When Sokka first figured out that Aang like liked his sister, he nearly twisted the Avatar's head off. It had been bad enough with Haru making googly eyes at her, and he had come close to ripping out Jet's guts when the slime-ball was putting his hands all over his sister. But Aang was supposed to be family, and family didn't like like other family, because that was just… gross. And sure, in a village as small as theirs it sometimes happened but dude, Katara was his sister.
Aang had been smiling at Katara whilst she went through one of her stupid exercises and Sokka thought that was just Aang being weird as usual when a slight blush crept up the younger boy's cheeks. Understanding had slammed Sokka in the chest as he recognised that expression as the exact same one he had worn around Yue. And Suki, for that matter.
Before he even realised what he was doing he was behind Aang, his boomerang raised. If Aang was even thinking about thinking about his sister in the same way Sokka had thought about Yue and Suki then by all the gods Sokka was going to smash his head in!
"Hey Sokka," Katara called over, "What are you doing?"
"Nothing!" Sokka said hurriedly, and lowered his boomerang. "Just … stretching. Can't let your muscles get cramped when you're a mighty warrior, you know." No, he couldn't kill the only hope for this world, even if he was making sheep eyes at his sister. Who was far too good for someone as dumb as Aang. Aang was a nice kid, but he thought asking a girl to come penguin sledding with him was a perfect pick-up line.
Urgh, had it been a pick-up line? He'd kill him for picking up his sister! And in front of her brother too! Did the kid have no shame?
"Hey Katara, look! No hands!" Aang shouted as he clung to the bottom of a tree branch using air. "Look, look! Katara, look!"
Well, that answered that question. Aang was still too much of a kid to think about his sister naked. Not that he should. No one should. It was not a sign of maturity but rather a sign of a man who was about to get his head smashed in by a very angry brother! But yeah, Aang was still too young to be thinking anything bad. It was probably just a harmless little crush that would burn itself out.
Still, he'd keep an eye on Aang, just to make sure. And if Aang even looked like he was thinking dirty thoughts, then it was the boomerang for him.