1. Install TwistedMexi's AllCheats script. 

In order to adjust your Sim's relationships using cheats, you'll need to use a third-party hack to enable certain in-game cheats.

  • Enter your game, open up Options, select Game Options, and click Other.
  • Enable mods and script mods by checking the respective checkboxes.
  • Restart your game for the changes to take effect.

2. Enable testingcheats.

 Press Ctrl+⇧ Shift+C, type in testingcheats true, and hit ↵ Enter.

3. Get the IDs of the Sims you want to marry. 

Open the cheat window, type in sims.get_sim_id_by_name [Sim's first name] [Sim's last name] and hit ↵ Enter.

  • The Sims need to be Young Adults or older.

4. Use the relationship Bit cheat. 

Type in relationship.add_Bit [Sim A's ID] [Sim B's ID] romantic-Married and press ↵ Enter.

5. Try the modify relationship cheat instead. 

If the relationship Bit cheat fails, you can use the modifyrelationship cheat to raise the relationship between the Sims so that you can manually make them propose.

  • Type in modifyrelationship [your Sim's full name] [other Sim's full name] 100 friendship_main and hit ↵ Enter. (Raising their Friendship status is necessary, as the proposal will be rejected otherwise.)
  • Type in modifyrelationship [your Sim's full name] [other Sim's full name] 100 romantic_main and hit ↵ Enter. This will raise their Romantic relationship, enabling them to start dating (which, in turn, enables proposal).