I have a family I already made myself plus 2 extra Sims I wanted to add to their family tree, and since it's an 8-Sim household, I had to split a few Sims from the 8-Sim household into the existing 2-Sim household of the new family members I created. No matter what, I can't make the Sim I want to be the other Sims' mother, or add the Sim I want to be their sister.
Edit By Crinrict: Adjusted formatting for readability.
@MJTiny Hello! I'm sorry that you're having trouble assigning relationships. Please check to make sure that the parents are older than the children; YA sims' parents must be Adults, Adult sim's parents must be elder, etc.
If that isn't the issue, then there is an existing bug report that may be related to your problem. At some point, the game appears to assign generic / unknown parents to sims. Please check your sims' genealogy / family tree to see if that is the case for you. If so, please click "me too" on the first post of the below report.
The workaround for that problem involves saving your household to the library and then deleting them from your game. This also involves using some cheats. Since you're trying to add relationships that affect more than one household, it is a little more complicated. Note that saving your sims to your library keeps all of their career / school progress as well as skills, money and traits. They will not retain any items in their inventories or relationships outside of the household. I did some testing and I think that the bug with generic / unknown parents being assigned may take some time to occur. If you're interested in trying some steps that may help, please click the below spoiler.
First, you may want to use the "save as" option, while the game is running, so that you keep your current save intact while testing these steps. You may wish to give it a different name like "Family Tree Testing" to keep track of which save is which.
Then, I suggest moving the sims back from the 2-sim household to reestablish the original 8-sim household and to preserve all the existing relationships. Once they're back in their original household, take them to CAS and save them to your library. If you want to keep any inventory items, go back to Live mode and place those items on the lot; this includes items in personal and household inventories.
After that, go to the 2-sim household and save them to the library as well. Also put any inventory items that you want to keep on their lot.
Once you have placed all inventory items you want to keep and saved both households to your library, go to Manage Worlds and delete both households from your game.
After deleting the households, select the lot you designated for the 8-sim household and use the "Create New Household to Move In" button in the lower right corner of the Map view. From CAS, use the Gallery / Library button to load the 8-sim household that you saved. Double check that all their relationships are correct. Then click the check button to have them move in. If there aren't enough household funds to buy their house, you can use the "freerealestate on" cheat to get them back into their house.
Once the 8-sim household is in place, click on the lot you designated for the 2-sim household and follow the same process to move them in.
Now use the Manage Households UI to split the sims you want to move from the 8-sim household to the 2-sim household. Close the Manage Households UI and enter the "cas.fulleditmode" cheat. Open the Manage Households UI again and edit the new 2-sim + transferred sims household. When CAS opens, edit the relationships as you wish. When you're done editing that household, then click the check button to play again.
At this point, load one of these households to play and check the genealogy / family tree panel to make sure the relationships between the two households are correct.
Please come back and update if these steps worked for you or not, OR if you have any questions about the steps.
