I saw your question posted last night, but didn't have the brain power to post an answer. In one of my current saved games I have my main family and their roommates who I don't control.
I used Nraas MasterController to add the roommates.
https://www.nraas.net/community/MasterControllerHere's my notes on a way to do it.
1. Split the family and move out the half you want to be your roommates into a different house.
When you go to add them as roommates and they aren't at home, you may have to go find them. Sometimes the game will tell you where they are.
2. Click on Sim you want to add > Nraas > M.C. > Intermediate > Add Roommate.
-- It will ask, Add the selected 1 Sims as Roommates. Click the check mark to confirm. It will reset the added Sim.
** In the Relationship panel, the roommates should have a little roommate icon on the left side of their profile photo. This is to show you that they are roommates.
Do this for each additional Sim and baby/child/teen you want to be a roommate.
3. *Optional* Click on City Hall > Nraas > M.C. > Town > Reset Everything. This will bring the whole family back together.