The same happened to me and it was really annoying. I believe the reason why it happens is some sort of bug with CAS.fulleditmode which I use a lot. I found a fix for it (after many hours of despair) and you need the MCC mod, because any other cheats won't work - at least they didn't for me.
With MCC, click on one of the sims you want to change the relationship and search for the tab "relationships" (if it doesn't show up on the first page, select next).
Then you'll have to delete ALL relationships that the chosen sim has with EVERYONE else. There's the option to "forget" too, but it won't work. So look for "clean relationships" or something like this.
Now they won't have any relationships with anyone - including family members. All you need to do is add relationships with family members.
It's going to be really tiring to get all your friends back (even with MCC), but at least that horrible bug is fixed. It's working perfectly for me so far.
I tried this and it worked temporarily.
Eventually, Bella Goth became my sims mother again.
Thinking the issue was that somehow the Sim was connected to Mortimer (who is already culled)
I deleted the affected the sims, created new ones, and at some point re-establishing relationships, Bella became mom again... Not sure what the issue is.
