The trigger is activated when you enter via the front door of the church with the marriage system unlocked, the vanguard system active, the command center is build, and Tabsy's big farm constructed.
Is the construction of the command center finished?
Did you finish a marriage or did you only start the wedding events but never go far enough in the events to properly unlock the system?
It's the nun in front of the altar in the main room, in case you're trying to talk with a different nun.