Ok this is an old post, and I still have not find a solution to this by someone else (I know the solution) which amazes me, and the CAS full edit mode DOES NOT work on this. (rarely it does with some relationships) I will post this guide to edit family relationships to an already created family, just in case someone would search it in the future:
1. Open the cheat console ctrl + shift + c and type: "testingcheats true"
2. Now type: "sims.get_sim_id_by_name yoursimfirstname yoursimlastname" (The desired sim must be selected)
-It will pop up a large number. This is very important. This number is your Sim ID
3. Type again the same thing in step 2 but now with your desired target sim name.
-I mean, if you want to change your (example) son and mom relationship, you need to type the text from step 2 from both of them
You should now have 2 large numbers in the console. (SimID)
---MAKE SURE TESTINGCHEATS IS ENABLED, AGAIN! OTHERWISE THE NEXT STEPS WON'T WORK---
-In step 4 you will write your desired relationshipBit. (Obviously I mean, don't write the word "relationshipbit") you need to overwrite the word with These relationshipBits:
*family_husband_wife
*family_son_daughter
*family_parent
*family_grandparent
*family_grandchild
4. In the console, type: "relationship.add_bit yoursimID targetsimID relationshipbit"
-YoursimID is the large number you got from your selected sims. You need to write the whole numbers.
-relationshipbit DOESN'T NEED TO BE WRITTEN, it needs to be one of the 5 commands I wrote in step 4.
If it doesn't work, it's because for some reason, Testingcheats disabled automatically (It happenned to me several times) Write it again and make sure its enabled again, and repeat the step 4.
Hope this helps.