I've added 'PotentialMarriageFaction' within the 'factions' tab under 'actor', but I'm guessing it's not quite as simple as that...?

Would I need to add anything within the 'relationships' tab at all?

Any help would be appreciated.

I think the answer lies in the follow package you add to the hireling. You might need to add one that points to the specific spouse... or the spouse alias if you're feeling brave.