No, Captain Sisay can't find cards that are only legendary when flipped
From the Comprehensive rules:
709.2. In every zone other than the battlefield, and also on the battlefield before the permanent flips, a flip card has only the normal characteristics of the card. Once a permanent is flipped, its normal name, text box, type line, power, and toughness don’t apply and the alternative versions of those characteristics apply instead.
Example: Akki Lavarunner is a nonlegendary creature that flips into a legendary creature named Tok-Tok, Volcano Born. An effect that says “Search your library for a legendary card” can’t find this flip card. An effect that says “Legendary creatures get +2/+2” doesn’t affect Akki Lavarunner, but it does affect Tok-Tok.