Responses
I thought it was the empress who said your niece since the chat bubble was completely round, indicating that someone else not in that panel talking. I get it confusing since the brother was the only one in box.
You're right. The biggest clue should have been how the emperor's brother repeated the word careful in the next bubble. Why would he repeat something he himself said...? I fail.
Okay one question. Did the emperor's brother really mean to say your NIECE or did he mean daughter? Because I'm pretty sure he knows the FL is his brother's daughter not his niece. And because no one seemed to find anything unusual about what he said. And because why did he say 'your' instead of 'our' if the emperor's brother meant his niece too instead of his own daughter then? And if he meant his own daughter how did he get around to knowing about all this stuff too? Was he told by the person he had a child with all this stuff so he wrote it down in a diary/journal like his sister-in-law did?