[ Katsa doesn't know what to make of all this pausing and uncertainty. She's already unfamiliar with reading people and their relationships; it's even worse that she can't see Zuko's face through all of this. It's obvious that there's something Zuko's not saying or doesn't want to say; but how much does she want to press the point? ]

Zuko. Why? Of course I'm careful. No matter how much she tries to tell me she's a "liability" without firebending, she's your sister. There'd be far more to her than that.

[ And she is Katsa. That speaks to an assurance of itself in her confidence. ]