When she was young, Kya had gotten into trouble the most frequently of Katara's children. Ever the curious little girl, she was much like her father; always playing, experimenting, searching. She and her mother clashed like swords on a battlefield, with Kya defiantly pursuing her every little whim and her mother left frustrated with her all too free-spirited attitude. Katara's daughter may have been a waterbender, but she had an airbender's natural talent for going against the grain.
Meanwhile, Zuko's daughter was purely the opposite. As steadfast and stoic as her mother, Izumi often left her father at a loss for words. It seemed he could do no right by the girl; when he approved her actions she would do the opposite purposefully, knowing it would not end well. Likewise, when Zuko voiced his disapproval of anything she did--a rare occurrence, as doting as he was--Izumi would immediately become determined to do exactly what he had advised her against.
Not that either young woman hated her parents; in fact, both loved them very much. But Kya had Aang's restless spirit and Katara's stubbornness, and Izumi was as determined as Zuko and as blunt as Mai. Needless to say, neither woman "calmed down" with age. If anything, they became more rebellious than ever before, thus why they kept their relations secret from Katara and Zuko.
...Or, so they believed.
"Did your daughter happen to mention to you that she and my daughter are in a relationship?" Zuko stirred his tea casually, watching his friend closely for her reaction. Katara's eyebrows raised slightly in amusement. "She did not," her voice held mild excitement. "Did Izumi tell you that?"
"Absolutely not," the former Firelord chuckled. "That girl even resisted telling me about her first marriage. Actually, Meelo told me." Katara smiled at the mention of her grandson. "Did he now?" Zuko nodded. "He asked if we were going to be related if Kya and Izumi married. I said no, but I actually have no idea." Katara laughed. "Well, we'd better not let them know we know," she winked. "Or they'll break up to spite us." Zuko nodded earnestly. "Oh, definitely."
Unbeknownst to the two friends, Aang's spirit sat between them, rubbing his temples.
"You two are ridiculous."