"She is my future wife."
Translation:Lei è la mia futura moglie.
Yes, the current given translation is wrong; the article must be omitted if the family member is a singular non-modified noun, and in this case the adjective modifies it.
Yeah, although "lei è la mia futura moglie" sounds better. That wouldn't be the case if instead of an adjective you used an indirect object, e.g. "lei sarà mia moglie in futuro".
Why the article la is used here? Besides the answer should be: Lei È mia promessa sposa. ALESSANDRO MANZONI
The article is only dropped before mio/a close relative when there's no adjective; either futura or promessa makes it necessary. If Manzoni drops it somewhere in his text, a modern Italian would correct him.
The published title is I Promessi Sposi. It's usual translation is "The Betrothed". While most future wives are indeed fiancées and so promesse / betrothed, English usage also allows "my future wife" to describe the speaker's intention to marry instead of a formal betrothal. So Duo's answer is a more accurate translation than yours.
A classic storyline is that a man says "I just met my future wife" as she leaves the room, and his life suddenly feels incomplete.