How did Alisaie know? [Before the Fall]

[Nalien]

From what I remember, it was implied she was with Urianger in the Waking Sands at the time, and there was suspicious Brass/Crystal Braves activity in Vesper Bay, enough to cause Urianger to cast a glamour on the Waking Sands, at least. Given how close her brother was with the Scions (not to mention Crystal Braves), she must have just figured something was happening when they came after Urianger, and sent the merchant to go check.