You might try this submenu from a sim's MCCC menu:
Sim:MCCC > Relationships
There are several options like adding or removing different kind of relationships like Mother/Father, Wife/Husband, Daughter/son or Girlfriend/Boyfriend. Please note that this is limited by the available game options, so something like Aunt/Uncle is not possible. Also note, that not every command is available on every sim. Some commands work on active sims only, some on NPC only. It depends on the game situation.
With these commands from the sim's MCCC menu
Sim:MCCC > Relationships > Set Active Relationship Levels > Set Active Friendly
Sim:MCCC > Relationships > Set Active Relationship Levels > Set Active Romantic
you can set the relationship level between your played sim and any available target sim ranging from Enemies to Best Friends/True Lovers. These commands work similar to the ModifyRelationship cheat, where you can set the relationship level from -100 to +100.
I haven't noticed any flaws so far in MCCC. Everything works as expected. I'm assuming that you use the latest MCCC version, which is v2024.2 at the moment.
Also these in game cheats from the command line
Modifyrelationship Sim01 Sim02 100 LTR_Friendship_Main (adding/reducing friendship level)
Modifyrelationship Sim01 Sim02 100 Romance_Mai (adding/reducing romance level)
work without problem in my game (v1.105.345).
