Hello! I believe this I am posting this on the right forum but I could be wrong so I apologize in advance.

Would anyone know how to manually set two sims as extended relatives such as cousins, grandparent/grandchild, etc?

I know these relationships are possible in the game but you need more sims to serve as "links" between them rather than just directly setting them as cousins.

Thank you in advance!

How I did it was to make the intervening sims for them to be related to and then use Master Controller to make he intervening sims dead, if that was the back story I wanted, or to move them out to the clipboard, if that was the story I wanted.

These are great ideas, thank you so much!
Does moving them to the clipboard get rid of them? Or will the clipboard sims still exist in the world?

I have never seen "cousin" listed in TS3. This does not show, even with Nraas MC, and the children of 2 sisters, which should be cousins.

NRass MasterController and the Cheats module is needed. Click on your active Sim you want to assign a family tie, NRass > Sim > Family > Choose accordingly Add Parent, Add Sibling and Add Child.

*Always back up your saved games and save first in case you mess up because NRass Family cheat can sometimes annoyingly misbehave and can cause unwanted familial connections regardless if you use the Remove Parent, Remove Sibling and Remove Child command. Therefore going back to just before you started messing with the family cheat can save time to re-do and restart.

To recreate cousins from a broken family tree you need a copy of the Sims in the family tree from the library. Whatever neighborhood you have the cousins, you put the copy of the siblings, their parents, there and then reconnect their sibling family tie if it's not already. Then link each parent to their child and the game will automatically assign the two as cousins. If you don't want the siblings, the cousin's parents, in the world I'm afraid it won't work any other way because the game needs the link in order to do create and keep the connection. Killing the copy of the Sims will not work and that will just reset as if no relation. If you don't want the siblings, their parents, in the world the only way I know of is to use NRass MasterController to make them a part of the service or role pool.

Click on Sim > NRass > Sim > Set Service (Pick whichever you think is best)

This way they will still be a part of the world but rarely be in it. You may have to reset them to send them quickly to the service/role pool after assigned.

Grandparents and grandchildren are the same, you need a copy of the Sims in the family tree.

Maybe this will help further: https://modthesims.info/showthread.php?t=666766
There's also NRass Traveler to connect worlds and keeps relationships but you may want to be careful about too much extra information that the game engine would have to remember which may cause performance issues.

Also, yes cousins exist in TS3.

Good to know! I guess it's the way that I play that I have never had the game show cousins. I have sent Sims to service roles before, so must try this to make cousins. )))

Thank you so much! This is really helpful and answers my question! :D