If this is a fiancée and the fiancée is Bianca Maria Sforza, then this portrait will date from ca. 1494.
From Wikimedia. Also posted to her new Subalbum.
Bianca Maria is dressed austerely in this portrait where the style closely resembles what she wears in this portrait.
Keywords: da Vinci, Bianca Maria Sforza, Queen of the Germans, Holy Roman Empress, deutsche Königin, Erzherzogin von Österreich, Kaiserin des Heiligen Römischen Reiches, Sforza family, Habsburg family, Empress, straight coiffure, caul, head band, pony tail, vest over-bodice, slashed over-sleeves, under-bodice, square neckline, under-sleeves