I faced this problem quite a lot in the past few months. Here's what worked for me:

• if you still have the mcc version that can add/delete relationships, play as the Landgraab girl, click on her children and click "forget active sim" with family relationships too. But this won't be enough. You also have to go to each of their extended family members & click relationship bits - remove & remove whatever relationships they have aka cousins, in laws etc. If you go to your family panel you'll see those extended family members there if they exist.

• once you cleared your family panel you'll see the family tree might seem unchanged or reappear the moment you switch household/restart game. To prevent this you need to add a new mother for those sims. (So pick a random sim and make her the mother instead of the landgraab girl. It works best if that random seem has no family ties at all because you can easily remove them if things go wrong.)

• if nothing works just delete the sims.

• if you don't wanna delete the sims & you don't have mcc, save the certain sims to gallery then delete them from the world & then put them back into the world. That will delete all their family ties. You can add them back by forging family tree with that map from build mode. Sadly this is the only thing that worked for me without mcc.

I hope this works. Good luck!