[NEEDS SAVES] Partnered Sims turn into siblings/family relationships

Product: The Sims 4
Platform:PC
Which language are you playing the game in? English
How often does the bug occur? Every time (100%)
What is your current game version number? Latest
What expansions, game packs, and stuff packs do you have installed? All expansion packs except Highschool Years, most game packs, a few stuff packs and no kits.
Steps: How can we find the bug ourselves? Using a save file from 2018
What happens when the bug occurs? My Sim A is engaged to Sim B, but when i check the relationship it is said to Sisters. The family tree also shows them being related. This is not possible, because i created Sim B in CAS, and Sim A is born in-game. They also have the Fated Mates relationship, because they are both Werewolves. They have two children together and two adopted children. When i open one of the childrens family tree, the grandparents on both sides are the same sims. Sim B is not supposed to have any parents.
What do you expect to see? Sim A and Sim B not being related.
Have you installed any customization with the game, e.g. Custom Content or Mods? Yes
Did this issue appear after a specific patch or change you made to your system? Neutral/Not Sure 

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Things i have already tried:

I tried editting the Household Relationships in CAS with cas.fulleditmode on, and i couldn't change them from sisters to household members.

I tried using the two Sims' sim id's to remove and add relationship with the relationship.add_bit {simID} {targetSimID} cheat, but nothing changed.

I tried copying the family by adding them to my library, making a new household in CAS and copying the family in, deleting Sim A and the children, so i only have Sim B. Then i moved the new Sim B in with the family, but they were still related. I did the same thing again, but this time i changed the name of Sim B, but still nothing changed.