Cas.Fulleditmode on, but can't make Gunther & Cornelia Mortimer's Parents, etc.

Hi
I’ve had that issue. What I had to do was completely delete the relationships between all the family members so they were all strangers to each other. Then I reset each family members relationship with each other using Relationships - Add relationship with sim on lot. You might need to cheat them as friends first and for couple relationships you might need to cheat them as friends and lovers first before adding relationship with sim on lot. There was one time this did not work and I ended up having to clear their relationships with all other sims before it would take.