Proclaim your kingdom as the Holy Kingdom

Once the marriage event is over, a few days later, you declare that your kingdom as a Holy Kingdom. It's still the same old kingdom with all the good and the bad, with another addition that the king will pick his wife from nuns. The reason that you give for the establishment of that cstom is that the wife will ensure that the king will never stray from the righteous path. She will be the beacon of hope for the people. Certainly not on a level to be canonized as that will go against the teaching of the Holy See, but a ceremonial rank given to replace the queen.

Not a Queen picked from among the nobles to rule the kingdom together with the king, but a Saintess elevated from the mass to work together the king in building a nation.