That's the point. Why someone who succesfully keep a secret during 20 years in the middle of a wild and very dangerous context… suddenly... confess that secret to (cough) an stranger.
Because she thinks her brother is in Wano. She knows that something is going on, that was the point of the whole banquet scene where Orochi is talking about the various incidents in Wano and relates them to Kinemon and the others reappearing. She thinks Zoro has something to do with them and potentially knows where Kinemon and Momo are, and she's not wrong. The time for rebellion is getting ever closer, of course it's the best time to now reveal the secret if you think Zoro is part of it.