In order to introduce the second character to the game (the "spouse" – a husband or a wife), you must complete the following tasks:  The Love Letter (5 parts) and the Wedding Hurly-Burly (5 parts). 
These tasks will be brought out to you as you move along the game-play,  completing other earlier tasks that lead on to starting a family. These tasks come up non-regardless of your character’s level in  the game – there is a certain set order for them.