My family tree keeps breaking

SnarkyWitch​ I'm so sorry for your frustrations! Unfortunately I doubt that the following will be of interest to you in your case, but I have had success with it in regards to a family I was deeply invested in the relationships of. I would suggest that if you play around with any of this you keep a backup of your entire 'The Sims 4' folder before doing so, and keep it in a safe place so if you don't like any detail of your results you can just add the old 'The Sims 4' folder back in and return to it as it is now. This is the 'The Sims 4' folder located in Documents > Electronic Arts > The Sims 4.

This is the information I had in a file from when I did it: Combining family and cheating relationships

Not sure what all was needed of this, but I separated all 8 Sims (including the child/children) and saved them individually into my library. Then...

1. In Manage World/World Map view I clicked "Create New Household to Move In" (lower right corner).  This opens CAS.
2. I left in the 'default, game-created Sim' and clicked my 'Open the Gallery' icon in CAS to view 'My Library', and chose one of my individually-uploaded Sims
   Sim > 'Placed Household' > Merge Household? > then choose "Merge". Now the Sim is in the new household.
3. I deleted the 'default, game-created Sim', then I proceeded to add all of the rest of my individually-uploaded Sims from my library in the same manner.
4. I made everyone but the two original parents of the adult children (plus they are grandparents to a couple of the household Sims) children. So the whole household was made of children and the two original parents (who are also grandparents).
5. Then I went through one at a time... made the two adults married and parents to their children (and checked for visa-versa).
6. Then I aged up their two children to adult. I made them parent to their children in the household (and checked for visa-versa).
7. Then I aged up the daughters-in-law and made them wives and moms to their children (and checked for visa-versa).

I went through then and double checked, that everyone had the correct relationship to each other. Though I had to re-add their traits for the most part, I think they still had their jobs and skills! Just cheat back the money the household had originally and anything else that's missing. (I would screenshot each sim's aspirations, skills, likes/dislikes, etc. before doing the whole process so you know if anything is missing when you're done.)

I think this way also creates a correct family tree in their Simology. (Just between the sims in that household you added, though.)

You could then add them to your game and see if all is working well. If it is, you could try adding the UI Cheats mod to re-add family relationships to others outside those 8 sims you could fit in the household you made by re-adding them from the library (as directed above), but those sim relationships added via UI Cheats will not be included in family trees. (I was told that's a less invasive mod, so I used it to re-add those additional relationships, saved, then removed the Mods folder and localthumbcache.package file and repaired the game before continuing, since I like playing without mods/cc.) I've had decent luck with them 'behaviorally' respecting the relationships I added that way, they can do 'family' interactions with them, etc. The most helpful/reliable relationship 'cementer' for me was by re-adding them from the library (as directed above) so they are in the family trees, though.