Am I sinful if I delay my fiancée's wearing of the niqab until after marriage?

A woman must cover her face from non-mahram (definition) men. Therefore, you should help your fiancée and encourage her to wear the niqab and not deter her from it, because it is a command from Allah. Allah Almighty says: "It is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should have [any] option in their decision. And whoever disobeys Allah and His Messenger has certainly strayed into clear error." And know that a suitor is a non-mahram to his fiancée, so it is not permissible for him to look at her, be alone with her, or shake hands with her until the marriage contract is concluded.