Obvious
Sokka decided to take pity on Zuko. It was just painful to watch their interactions at this point.
"Zuko, buddy, let's chat."
The Fire Lord looked up from the pond where he had been holding his head in despair and moaning about his unrequited love to the poor turtleducks.
"Uh, sure Sokka." He gestured to the ground next to him.
Sokka cut to the chase. "I know you have a crush on my sister."
Zuko flushed red. "What? No I-"
"Shush." Sokka waved his arm theatrically. "Don't deny it. It's very obvious, my dude."
The fire bender only groaned, slumping back onto the tree behind him. Sokka continued, "If this were anybody else, I'd tell them to get lost. But, you're...decent. And… well, I know that Katara likes you a lot."
Zuko's head spun to him, gaping like a fish out of water. "Wait she does?"
"Obviously." Sokka rolled his eyes. "You two are helpless."
Sokka sighed as Zuko continued to stare at him in astonishment. "Look, just ask her out already. She'll say yes."
"I… okay. Thanks, Sokka."
Sokka patted his shoulder and stood to leave. He muttered, "I can't believe I'm subjecting myself to oogies for those two idiots."